Do you associate letters with colors? Has the letter A always been yellow in your mind? Do sounds trigger a certain taste? If so, you experience a phenomenon known as synesthesia.
It's said that this occurs because of crosswiring in the brain. Different senses become connected to one another, so when one is triggered, another is simultaneously. It's really interesting!
For me, letters and numbers have always triggered a specific color in my mind. It's been that way as long as I can remember. Days of the week and months trigger a color as well. January is dark purple in my mind, for example.
This tends to be more common among us spectrumites. I've talked to others who have similar experiences. The brain sure is a fascinating thing!