According to Coldplay, recently lofted bafflingly high in the UK charts, 'It was all yellow'. Quite what it was and how it came to be yellow are not specified, although several unsavoury suggestions come to mind by the time of the fourth hearing of this song (ingeniously entitled 'Yellow'). However, one of the many yellow items that appear in this indie dirge is a 'song', which introduces us to the fascinating phenomenon of synaesthesia, the merging of the senses. A person can experience a sensation through a sense which is not being stimulated. For example, a person might hear a sound but experience it as a colour.

This is not as freakishly uncommon as one might think, especially among composers. Rimsky-Korsakov saw the key of C major as white and D major as yellow. His lush, swirling yellow is obviously a different shade from Coldplay's colour, which is denoted by a scratchy guitar and vocals like a dog in a cider press. In the infinite variety implied by this dichotomy lies the whole problem with synaesthesia; it is extremely difficult to communicate precisely a synaesthetic experience to a second, objective person, as we may all see, or hear, colours differently. Coldplay may be seeing yellow, but I generally observe a red mist descend before my eyes whenever they come on the radio.

In the late nineteenth and early twentieth centuries there were attempts to fix the relationship between sound and colour mechanically. Remington's Colour Organ and Wilfred's Clavilux were both keyboard instruments that, rather than producing sound, emitted different coloured lights corresponding to different pitches or keys. Both Scriabin and Schoenberg wrote pieces for the Clavilux although, understandably, they have never been popular amongst the record-buying public. Indeed, whether anyone beyond people who like to watch traffic lights would actually want to see these works is very much open to question.

Sir Arthur Bliss's response to his own profound synaesthesia was far more audience friendly. His Colour Symphony of 1922 attributes to each movement a colour, depending on its key, and then explores the heraldic significance of the multifarious hues. For example, the first movement, Purple, is Majesty. The resulting symphony is loud, expansive, and also rather long; one does wonder whether Bliss might have had an overarching Grey theme running through the work. Nevertheless, his is an expressive and communicative approach to his very subjective experience. Opening night stories of a colour blind conductor confusing the movements and conducting Green Majestically and Purple with a rather Reddish sense of legato are largely apocryphal.

The conflation of the senses is by no means limited to the world of classical music. Synaesthesia runs throughout literature and the visual arts. As far back as the seventeenth century the poet Andrew Marvell wrote of thought itself as a colour:'A green thought in a green shade'. In twentieth century literature, we find a notable synaesthetic in Vladimir Nabokov. In film, Derek Jarman's 'Blue' attempted to provoke a synaesthetic response in the viewer. The screen remaining a blank shade of blue for the entire length of the film, a soundtrack of 'blue' noises is played. The result is either hypnotic, sedative or aggravating, depending on one's attitude to watching a bare wall while someone forces you to listen to seagulls.

In pop music, synaesthetic effects are wrought less self-consciously but no less commonly. 'Blue' is, plainly, a colour that has become conflated with a sound. This colour has given rise not only to the Blues, but also to such evergreens as 'Blue Moon', 'Blue Suede Shoes' and 'Blue Velvet'. 'Blue' creates a sensation of melancholy and comes to signify far more than the simple phototropic effect of the colour itself. The 'blue' note, the 9th interval, is an aural denotation of the colour and its concomitant emotions which works independently of any visual stimulus. In the case of Gershwin's 'Rhapsody In Blue', it is a key in its own right.

When one considers that Blind Lemon Jefferson was one of the foremost bluesmen, it seems that it is not necessary tosee the colour blue to understand its cultural connotations; it is a state of mind. Synaesthesia becomes easier to understand because it is now obvious that notions of sense and perception are easily mingled in the brain. Senses overlap so that we can to a certain extent hear colours and see noises, which makes watching ('wacky' kids tv presenter)Timmy Mallett on TV both blinding and deafening even with the sound turned off.

It is important to distinguish between pure synaesthetic effects and the use of colour imagery.The Beatles were possibly the first pop group to apply colour imagery consistently to their songs, but did not achieve synaesthesia. Their early works are full of references to 'blue' moods, and a causal relationship between different colours. Let us take, for example, 'Yes It Is', in which 'Red is the colour that will make me blue', or 'Baby's In Black': 'Oh dear, what can I do/ Baby's in black and I'm feeling blue'. An extraordinarily complex colour scheme means that both red and black can lead to blue, while blue leads to nothing but depression.

Red and black mixed actually give rise to a purplish sort of colour, which suggests that maybe John Lennon, who wrote theselyrics, was colour-blind. This disability may have diluted the synaesthetic properties of his mind for, while the blind person has no visual preconceptions and may sense colour on some subliminal level, the colour-blind must view the world through a layer of confusion. While we may all see colours differently, the colour-blind just get them plain wrong. Consequently, Lennon could never achieve a purely synaesthetic effect. He never quite put his finger on the blues, although he constantly strove to attain them. Moreover, green was a totally inconsequential colour for him, as it is for deuteranopic colour-blind people, who see green as grey: 'You say you've seen everything there is/ And my bird is green/But you don't see me'. (And My Bird Can Sing).

Green doesn't fit in to any scheme at all while blue is of utmost schematic importance. The genesis of this blue-green duality can be found in the old rhyme, 'Blue and green/Are never seen/Except with a colour in between', often used as an aid to snappy dressing by the deuteranopic. Lennon probably had to rely on such rhymes to inform his clothing choices, in view of his colour-blindness, and the colour imagery in his songs was subsequently influenced by them. This didn't help anyone when he asked George Martin to make one of his songs sound 'orange', which would have been pure synaesthesia but for the fact that he was probably thinking of 'green' all along. We can only guess at the confusion in his acid-battered mind when Paul McCartney proffered 'Yellow Submarine' for his approval. Lennon's lukewarm attitude to the song may be explained by the inclusion of the line 'Sky of blue/And sea of green' without another colour inbetween.

The Beatles were not the only Sixties act to indulge in colour imagery without ever reaching a synaesthetic climax. Joni Mitchell treated us to her idea of 'Blue', which seemed to consist of a strange birdlike warbling juxtaposed with the random tinkling of an untuned guitar. This is not so much blue as sort of khaki with beige flecks. The Rolling Stones used colour rebelliously in their glorious 'Paint It Black' and did, indeed, make a sound resembling black music, but this was less a case of synaesthesia than of politely borrowing the entire musical heritage of the Mississippi Delta.

Jimi Hendrix did his damnedest to make his guitar sound like a 'Purple Haze'. The result is, indeed, majestic; obviously Hendrix and Arthur Bliss agreed on the dramatic connotations of purple, although Bliss was unlikely to be seen setting fire to his conductor's baton and throwing it into the crowd in quite Hendrix's style. Yet there is little to suggest that Hendrix was attempting to sound specifically 'purple' in his choice of key or pitch. His aim was, ultimately, impressionistic rather than strictly synaesthetic. That the impression he created was of a hundred and fifteen crazed goony birds chasing their errant brains around a prairie is irrelevant to the colour 'purple'.

The master of synaesthesia would, strangely enough, emerge from the estimable band whom Hendrix supported on their 1967 tour - The Monkees. Mike Nesmith first came to prominence as the woolly-hatted one who wrote such glorious songs as 'Listen To The Band' and 'The Girl That I Knew Somewhere'. He became well-known for his deep Texan twang and for the ludicrously tight striped trousers that he wore in the promotional film for 'Daydream Believer'. Whether these were designed to conceal and cure an unpublicised hernia or were a loan from a heron's outfitter remains uncertain. As part of the plastic mop-top foursome he participated in the non-synaesthetic use of standard colour imagery, notably in 'Shades of Grey' and 'Early Morning Blues and Greens', neither of which were written by Nesmith. Both of these songs display the same deuteranopic colour scheme which John Lennon had used, which just shows how desperately The Monkees' creators were trying to emulate The Beatles. After The Monkees fell apart in the late Sixties to a clamour of public indifference, Nesmith followed his own muse down the road of whimsical country-rock balladry and was, in turn, followed by absolutely nobody at all. His solo oeuvre, nevertheless, is impressive in that it does include the most purely synaesthetic song in the pop canon : 'Rio'.

'Rio' was Nesmith's only solo British hit, reaching number 28 in March 1977, and also accompanied Kenneth Branagh's onanistic thesp chum movie romp 'Peter's Friends'. Despite, or perhaps because of, its exposure in Branagh's grimly emetic film, the song has never received due recognition of its synaesthetic qualities. Nesmith deftly avoided the difficulties of describing colours in sound by referring directly to the senses themselves, boldly laying down his synaesthetic cards in the first two lines : 'I'm hearing the light from the window/ I'm seeing the sound of the sea'. Like the lamp which 'mutters in the dark' in T.S. Eliot's 'Rhapsody on a Windy Night', the moonlight and the waves are sensual vehicles of Bergsonian time, in which objective reality is subservient to subjective experience: 'My feet have come free from their moorings/ I'm feeling quite wonderfully free'. While Eliot's synaesthetic lamp betokens rather damp and dreary memories, however, Nesmith's imaginative journey is joyous: 'I think I shall travel to Rio/ Using the music for flight'.

Synaesthesia, the conflation of the senses, provokes in Nesmith an out-of-body experience as he astrally projects himself to Rio de Janeiro which is, plainly, a more attractive scenario than T.S. Eliot lurking in the streets of London in a dank raincoat worrying about his mad wife. Not known for his samba talents, the esteemed poet and erstwhile bank manager never explored the liberating qualities of synaesthesia, while Nesmith was so impressed by the whole experience that he invented MTV and then sat back to reap the dividends of inheriting the patent for Tippex from his mother.

It is also important to note that, when Nesmith used colour imagery, he was, like Lennon, incapable of reaching a synaesthetic climax. In his song 'Joanne' Nesmith returns to the deuteranopic norm. In a peculiarly opaque verse, he sings that his wisdom ' broke down' Joanne's desires like 'light through a prism/Into yellows and blues and a tune/ That I could not have sung'. Yellow and blue mixed make the colour green. In these lines, green synaesthetically represents a tune that Nesmith cannot articulate; once again, deuteranopic colour-blindness has smothered intersensual statement in its infancy. This only serves to emphasise his artfulness in sidestepping the whole colour problem and heading straight for experience itself in 'Rio'.

A performer who has sidestepped colour through necessity is Stevie Wonder. Blinded in early infancy through an excess of oxygen in his baby incubator, Wonder has no concept of the conventions of colour and the visual world (as evinced by his choice of multifariously hued kaftans). This, however, has not handicapped his sense of the visual. Rather than being constrained by colour, he is impelled to create a synaesthetic world of vision in sound. Indeed, on his album Innervisions, in which he explores ideas of seeing, the song 'Golden Lady' creates an uncannily aureous sonic atmosphere. His sense of what it is to be shiny, metallic,yellow and also slightly tasteless is expressed through the awesome power of the Moog synthesizer; close your eyes and you can almost hear a lovely glittering nugget. In the shape of a lady, obviously. It is eerie that a man who cannot see should be able to suggest through sound alone the image of gold to those of us who are sighted.

The Eighties did see various performers attempt to maintain the synaesthetic approach to music. The most notable example is ABBA, whose 1981 album The Visitors featured a song that had learned its lessons directly from Mike Nesmith. 'I Let The Music Speak' appeals directly to the synaesthetic core of the brain to express the transformative power of song : 'I'm hearing images/ I'm seeing songs/ No poet has ever painted'. In the same year, The Human League pressed the 'synaesthesia' button on their Roland synthesizers to produce the song 'Darkness' on the album Dare! : 'I hear colours/ Black and red/I see sounds/ That fill my head/ I'll never read those books again'. We cannot know which books Phil Oakey was warning us against, but we should assume that they were by Harold Robbins.

1981 was clearly a highly synaesthetic year, perhaps as a consequence of the dizzyingly peculiar union of Charles and Diana, which looked fragrant but smelt strangely pear-shaped. Sensual confusion was as clear in the year's music as it was in Diana's inability to say her husband's name correctly at her own wedding. Thereafter, the story is familiarly deuteranopic. Cyndi Lauper made a brave stab with 'True Colours' - 'I see your true colours shining through/Your true colours, and that's why I love you' - although this song fell at one important hurdle. It did not specify exactly what the 'true colours' were, except that they were 'beautiful like a rainbow', which is, of course, transient and ephemeral. This is no basis for a loving relationship and anyone who tells you otherwise is fundamentally unsound.

The band Colour Me Badd introduced an interesting idea of colour as a moral quantity. In this case, our moral sense is confused with our sense of vision,a kind of ethical synaesthesia. Colour and image become evil and trustworthy, a suspicion backed up by the sight of the band's terrible red jackets. Madonna inverted the moral quality of colour in her song 'True Blue', in which colour signifies ethical probity and integrity, and also ponytails, bobbysox and hand-jiving. Various Goth bands performed various songs which variously ascribed qualities as various as death, depression, mortality, gloom, the shuffling off of the mortal coil and feeling a bit miserable to the colour black, but any hope of synaesthesia was quashed by the uniformly grey noise which emanated from them.