The phenomenon of experiencing associated senses (a kind of cross-wiring) is an old concept, but it has only been scientifically studied since the second half of the 20th Century. This condition causes a deep emotional response in the people who experience it. Affected people think that they are crazy, as do other people who do not share the sensations. However, synesthetes love their perceptions, and they mourn its loss if they grow out of it. It can also serve as a memory aid because people can remember the colors and then work out the words that they are striving to recall. 

Okay, what exactly is synesthesia?

It is almost like a narcotic drug trip, yet the people are not using psychotropic or other drugs. Supposedly, it affects 4 people out of 100,000. There are different types of synesthesia, and different senses that can be interconnected. For example, touch and vision, vision and hearing, etc. People with synesthesia attach certain letters to certain colors. T's might be brown for one synesthete, yet pink for another. It depends much on the person. There is no cut-and-dried guide to synesthesia and colors. However, even as they see certain letters as being associated with certain colors, they realize if the print is black-and-white.

The associated colors can change over time, and people can lose their synesthesia at any point during their lives. People can also gain synesthesia as a result of medical trauma, such as a stroke or tumor. Many synesthetes also experience colors as associated with sounds. For example, when they hear music, they see colors in their vision field that most people do not perceive. 

Synesthesia (and bipolar disorder) are said to be associated with creativity in the arts. There are many famous people who have synesthesia, including the following: 

• Itzhak Perlman is a violinist. Most people can appreciate the association of color with music. For example, blue may be associated with more somber tones, and red with energy.
• Sir Robert Cailliau is an engineer who helped develop the World Wide Web.
• Billy Joel is a singer/songwriter who dreams in the abstract. He composes songs in his sleep, then he recalls them by their colors.
• Norman Mailer was an author.
• Stevie Wonder is a blind piano player.
• Douglas Coupland is a novelist and artist who obtained synesthesia through a health problem.
• Brit Daniel Tammet is a novelist who has Asperger's Syndrome.
• Vincent Van Gogh was a painter.

Tasting the Universe: People Who See Colors in Words and Rainbows in Symphonies by Maureen Seaberg, 2011

The author has synesthesia.

• Books about synesthesia
• American Synesthesia Association