Actually the protagonist dragon in my book is black and red mix, he breaths multiple elements, and he is not "evil" (well not totally good either)
Color of dragon is what everyone mentioned here before, based on the original Dungeons and Dragons conception that the color indicated alignment (good / evil - Lawful / Chaotic) and their breath weapon.
Age determined what magical abilities they had access to (based on mage "per level learned" slots) and natural abilities. Things have changed between 1.0 and 3.5, dragons are a lot more nasty in 3.5 rules.
Color, from my perspective, is interpreted by your individual imaginations and followings of what informational source you tend to read.
To me... red dragons don't always breath fire, green dragons don't always spit acid. Color isn't like Anne Mcaffery's description of pigmentations and such either (but if you are a follower of her works, this is how you would understand it.) A dragon's color is what YOU make it based on your understanding (based on novels or RPG games) of how colors work and not everyone will be on the same page.
Also, when creating dragons for fiction and fantasy, you wouldn't want your dragon to have a dark and shadowy name, only to make him/her white and bright in color. "Hi I am EbonStrike the dragon, check out my totally white scales!" just doesn't sound good