The color of magic for Dragon Riders matched the color of their bonded dragon. It is not known how other magic users get the color for their magic, although it is likely symbolic to that magician, or in some other way matches a quality of him or her. This color is not always displayed when magic is used by that individual, but it will usually appear when magic involving fire, light, or other forms of direct energy are used.

Color discussed in Eragon Edit

Eragon Brisingr Bow

Eragon, in the movie, using Brisingr on his arrow

Brom and Eragon discuss the color of magic in Eragon:

"Why was the fire blue? How come it did exactly what I wanted, if all I said was 'fire'?" - Eragon
"The color varies from person to person. It depends on who says the word. As to why the fire did what you wanted, that's a matter of practice..." - Brom

Eragon's magic was blue and matched the color of Saphira. Murtagh used red magic because Thorn was red. Coincidentally, Arya used green magic throughout the entire series, even before she was bonded to the Green Dragon Fírnen. It is possible for magic users to briefly change the color of their magic, but the only point to this would be to evade detection, as it is assumed that the colour of magic has no effect on the results it produces. It may also be possible to have people who have white or black magic, even though they are not really considered colours. Vrael most likely had white coloured magic, considering that Umaroth was a white dragon, and Shruikan, a black dragon, had Galbatorix not interfered, would have had a rider that would have had magic that was black.