The experience of colour as we usually understand it is a visual one: objects have colour, artists use colour, and we can recall a colour in our mind’s eye. But for some people, colour is a more multi-sensory affair, linked to sound, texture, taste or shapes. Music has a hue – like the parping of a trumpet that evokes a shower of burnt orange. Numbers, letters and days of the week have their own shade: the number one is white, the letter L is blue and Monday is red.

This neurological phenomenon is called synaesthesia; if you don’t have it, it sounds strange, like the straining of an overactive imagination. But if you're part of the estimated four per cent of the population who are synaesthetes, such descriptions are as obvious and natural as the sky being blue and the grass being green. Synaesthesia is best described as a union of the senses; one sensory experience involuntarily, and consistently, prompts another. There are up to 70 different types – from tasting the time to smelling a symphony – although the most common involve colour.

I have grapheme-colour synaesthesia. Numbers, days, months and letters (and by extension, words) have their own very distinct shades. The examples above are my own: L could only ever be blue. This is the most common form of synaesthesia, and it wasn't until I was in my early twenties that I realised not everyone sees the world this way – indeed, the assertion that words have a certain colour can be met with sneering scepticism. Yet I simply know that Wednesday is dark green, for it always has and always will be. Talk to another synaesthete, however, and they’ll often disagree with you howlingly about the colour, or even the precise shade.

One friend has tastes and textures as well as colours for words. While I find my synaesthesia has little impact on my day to day life, for her, it’s a curse as much as a blessing. There are words she can’t say without physically cringing. For others, however, synaesthesia can actually be a bonus, aiding their creative endeavours. Research on synaesthesia is not as extensive as you might expect: it was disregarded as a phenomenon until the advent of MRI scans in the late 1980s, proving that corresponding areas of the brain really do light up in synaesthetes – but it’s thought it may be more common in artists.

The art of noise

Synaesthesia has been something of a hot topic in music news recently, with the likes of Pharrell Williams, Kanye West, Lady Gaga, Dev Hynes and Frank Ocean suddenly keen to talk up their colourful experiences. Pharrell’s international mega-hit Happy, if you were wondering, is yellow, with accents of mustard and sherbet orange.