According to Mirth (Kevin's first Mage), Magic is a River and it's color is the Color of the Faerie Realms -- Green.
As the World-Mage, Mirth's connection to Magic itself was completely pure and for him it's Color was Green. For others however, the Color of Magic is altered by their influence. It might be Red (as seems the case of the Pale Incanter). The Color of Magic could even be Purple.