Synesthesia: Colour associations with other senses
Do you ever associate certain letters with certain colours? Or certain smells with certain colours? Well, there are people who do.
It could be the correlation of a particular object, a specific kind of odour, a food item, a loud noise, or anything else, with a colour. This happens because sometimes the brain draws correlations between two different sensations - Synesthesia (as opposed to anesthesia, which is the absence of any sensation).
It's fascinating, really. For example, these people will always see the letter 'A' appearing in red, even if it is printed in black on white background! Their brain is on overdrive, especially because there are multiple things happening at the same time - seeing what the object is for what it is, associating it with a colour, remembering earlier instances and similar associations and so on. As a result such people seem to be able to remember things much better than others.
Synesthesia is therefore the involuntary triggering of one sensation when you experience another sensation. Chromesthesia is the specific term to describe the association of a particular colour when you experience any other sensation.