It is called synaesthesia, which refers to a variety of different conditions. I have one of the most common types of synaesthesia (and I have never taken any psychedelic hallucinogen). It's known as color-grapheme synaesthesia. What this means is that I often "see" letters and numbers as having certain colors. Like for example, if I glance quickly at the letter A there is a red tint, and the letter A strongly reminds me of a deep red color whenever I see it. Every letter and number (plus some punctuation like the slash and the question mark) has its own distinct color associated with it in my head. Words will often take the color of their first leter. For example, the word "color" looks yellow to me since C is a yellow letter. It's not as interesting as hearing colors, but it's pretty neat nevertheless.