Some people see colors in letters or numbers called color-graphemic synesthesia, see colors from sounds call sound-color synesthesia, or taste from hearing or saying certain words call lexical-gustatory synesthesia.
How do some people have this ability?
One of the beliefs argues that all people initially have this ability, but because of later stages in neurodevelopment when neuronal pruning occurs people lose the ability. The individuals who do not go through this process completely may have synesthesia.
Other arguments for how one can have synesthesia include cross-wiring of neurons, or atypical functioning of the ‘association area’ in the brain. Synethesia specifically color-graphemic can be induced through taking hallucinating drugs such as Magic Mushroom.
Most people see the image on the left, but individuals with color-graphemic synethesia see what’s on the right. 5’s are green and 2’s are red. People with synesthesia has been linked to having stronger memory.
Here’s also a YouTube Video on this: http://www.youtube.com/watch?v=veoN1mh7RME