I have synaesthesia. A very mild and common form known as Grapheme → color synesthesia. This means that when I see or think of letters, numbers, or types of words, I "see" colours in my head.
Well, in my "mind's eye". I don't physically "see" the colours (although some Synaesthetes do). When I look at words on a page, they remain the same colour as the ink they are printed in, but in my mind I "know" what colour they are.
I have always had this association between colour and word/letter/month/day, as long as I can remember, and in fact when I was very young I assumed that everyone knew that Monday was red, and Tuesday was blue, or the number 8 was a light greeny-yellow. It is common for synaesthetes to find out as children that the rest of the world doesn't share this knowledge, and that their friends look at them strangely when they say that they prefer certain months because of the taste, colour, or smell. We quickly learn to keep quiet about how we see the world.
So how do I see the world?
Well, a lot of the letters of the alphabet have colours, all of the 10 numbers do, all of the days of the week and all of the months.
Not all of my letters are coloured but all of my vowels are, but although some of my letters have reddish colours (R, B, P for example) and some are greenish (F, K), the overall colours of words tend to be defined exclusively by the colours of the vowels...and I have no red or green vowels.
Of course, to confuse things even more, days have colours which are different to (and override) the colours the words produce! Thus the word "Monday" would be black-blue, but the day Monday itself is Red. The same thing occurs with numbers. For example, the number 5 is Red, but the word "five" is white-yellow!
How does this affect me on a daily basis?
Well, not adversely. The colours of words, letters and numbers just tend to be "there" alongside everything I do and think. Although underneath it all, I have come to realise that the colours of my days tend to strongly influence what days I like and dislike, and what I do on those days.
For example, Wednesday is my favourite day, because I like the colour. Friday is a good day because the colour is the most vibrant (Lime Green). Tuesday and Thursday are almost interchangeable, and I have great difficulty making plans when I have to choose between Tuesday & Thursday.
In general then a word will have a colour (mainly defined by it's vowels). This led to an interesting realisation that whereas for most people words have two characteristics - how they're spelt and what they mean - for me they have a third - what colour they are.
This was suddenly noticeable once when I was trying to think of a particular word but couldn't. You know, when it's on the "tip of your tongue" but you just can't remember it! Well I knew what the word meant, and I could think of words with similar meanings...but I also knew the colour of the word (it was yellow). However the word itself still eluded me. It was as if I was staring at a word-shaped hole in my mind, but with a kind of sign that said "This word is yellow".
And then it popped into my head (as these things do). The word was resident. And I was correct. it was yellow. What this says about the way my brain works I don't know, but it's as if I have an extra dimension to my vocabulary.
And then there's the months
Like days I have an affection for certain months because of their colour. The "warmer-coloured" months are my favourites, like March, August or September. Interestingly June and July seem to have a similar relationship to Tuesday and Thursday. They are the same colour, although slightly different shades, and from speaking to other Synaesthetes this isn't unique to me. Other people have had the same colours for those two months and those two days.
How to identify the Synaesthetes in a room
This is a good one, and it works. Next time you're with a group of people (this could be friends down the pub, a group you're teaching, or a lecture you're taking), get their attention and then quickly ask them this question:
"What colour is Tuesday?"
Any synaesthetes in the room will answer first, automatically, and without thinking. If anyone has to ask "What do you mean?" then they're not synaesthetic. Grapheme → color synaesthetes know what colour Tuesday is, and they always have.
How do I know what colour a word is? What does that even mean?
Ask a non-synaesthete this question: "What colour is the sky?" They will immediately answer "blue". But the sky isn't blue. It's grey or black, or dark red, or whatever colour it happens to be at the time you look. So why do we say it's blue? Because that's a characteristic that our language has attached to the word "sky", and everyone now knows it. Similarly "what colour are trees?" will generally produce the answer "green".
Well that's how I see the world. The sky is Blue. Trees are Green. And Monday is Red.