I was 30 and a mature student in art college when I first realised I saw the alphabet in glorious technicolour. I was building a 3D calendar (three wooden cubes telling the day, date and month) when my tutor asked why I had painted the six blue. “Because that’s what six is,” I told her.

I think of my grapheme-colour synesthesia as a curiosity, rather than a condition. I see every letter and number in its own unique hue and, when they are combined into words, every word makes a colour that is equally unique to my mind. Experts say it is a blending of the senses; to do with the way signals in the brain are processed.

But it is impossible to recognise that you have it until you realise others don’t; it was not until that odd moment in the art studio that I started to question whether a rainbow alphabet was the norm.

After the conversation with my tutor, I started to talk to people about it, but they didn’t understand and I soon stopped because they clearly thought it was nonsense. Synesthesia of one sense or another is estimated to affect up to 0.5% of the population and can present itself through taste or hearing music, too.

I can tell someone the colour of their name instantly. Processing words is like mixing paints on a palette: C is white, D is beige, E is yellow and so on. They have never changed or reconfigured.

It was my youngest daughter, Jenny, who was most intrigued by it. She found it magical as a child. As an adult, I remember buying her narcissus flowers because they were the same yellow as her name.

My other daughter, Katie, is a metallic blue-black and my son Mike’s shade is just a bit lighter – they both have a K, an I and an E. My husband, Bob, is brick red.

When Jenny told me the names of my granddaughters, each time it was their colours I saw first: Lila is white; Ava, her little sister, is green.

I also have a degree of spatial synesthesia, which means that days of the week, months, years, follow a pattern in my head; a sort-of curve. If you ask me what 10 years looks like, I will immediately zoom out from a week, like a camera lens.

The associations are involuntary but, in many ways, they are rather like having particularly pleasant muzak on in the background – the colours are always there. As a furniture designer, it did not have a tremendous bearing on my trade. But in daily life, words sing out to me everywhere I go. Advertising hoardings, road signs and posters are all brighter and louder to me. If I zoomed in and allowed myself to see every single colour, it would be overwhelming.

I read the papers every day and about four novels a month, so it certainly hasn’t distracted from my enjoyment of the written word. If anything, it enhances it. The Guardian, incidentally, is a mixture of pinky-beige and green.

Synesthesia is not a definite science. I remember reading someone else’s story in a book, thinking their colours were different from mine. Each synesthete experiences their own personal colour spectrum.

Even now, at 65, it sometimes surprises me. My friend and I are writing a book and we’ve had to Google a lot of things: new words mean new colours.

I can’t imagine what it must feel like not to have it, except that it must be rather boring. In some ways I wish I’d known earlier, but I was brought up in the 1950s when parents were inside the house and children played at the bottom of the garden, so no one sat down with me and asked why I’d chosen the colours I had when I did a painting.

I have heard that it is possible to train people to see the world in this way – and that it might help with dementia. But it hasn’t affected my memory. In fact, Bob will ask me someone’s name and I will say to him, “I can’t remember, but I know it’s green.”

There is nothing dull or grey in my spectrum. I like the colours that I have created, but that blue number six is still my favourite. That one has always made me smile.

• As told to Deborah Linton

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