I too see words in colour. Not just the days of the week and months of the year, but people's names, numbers and, well, anything really. For instance Susan is yellow, Rebecca is green, James is dark blue, Ian is light blue. Monday is a kind of rusty red, whereas Wednesday is a dark red. Tuesday is a sort of silvery colour, Thursday is cream. Friday is orange.
Synaesthesthia seems to be a 'thing' in my family - my mother had it, as does my sister and my son. Those without it don't know what the heck I'm talking about
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I've often wondered whether the colour thing links to very early learning, where our first reading books were brightly coloured and the alphabet letters would all be individually coloured. Is it this that suggested words should be represented by colours? Not necessarily the colours we saw in those first reading books, but the concept may have originated with them.