It's called synesthesia, and it is the neurological condition in which sensory information is misconstrued in the brain and interpreted differently. This results in involuntary experiences of a second sensory pathway that is not supposed to be triggered.
A common example of synesthesia is known as grapheme. This is when letters or numbers can be perceived as colors, tastes, forms, or even to have personalities! A person with this affliction experiences all five senses simultaneously when they should be separate.
Teri Floyd described the number 5 as "blackish gray, and in his early 20's." Genders, textures, and ages can be attributed to different numbers also, and sufferers often describe the experience as overwhelming.
It is the result of the brain failing to fire neurons at the correct time, so when they see something, instead of just seeing it they feel it and hear it too!