I visualize individual numbers and letters in specific colors and spatial representations. Both of these types of mental representations are forms of synesthesia, and were first described by Sir Francis Galton in 1881, in “The Visions of Sane Persons” and “Visualized Numerals” and has been the subject of several psychological studies in recent years*. The spatial representation of numbers in the mind is called a number form. My friend Matt has a longer description over at his blog, Many Worlds Theory. Since mine is mainly a photographic blog, I will simply illustrate:

*Noam Sagiv, Julia Simner, James Collins, Brian Butterworth, Jamie Ward, What is the relationship between synaesthesia and visuo-spatial number forms?, Cognition, Volume 101, Issue 1, August 2006, Pages 114-128, ISSN 0010-0277, 10.1016/j.cognition.2005.09.004.
(http://www.sciencedirect.com/science/article/pii/S0010027705001551)