From the MM:
Known varieties of Dragons fall into one of two catergories: chromatic and metallic. Chromatic are black, blue, green, red, and white; all evil and extremely fierce.
Metallic dragons are brass, bronze, copper, gold, and silver; all good, and usually noble. (p 61)
Present dragons keep to themselves and only cooperate under extreme circumstances. Good dragons never associate with evil dragons, though there are exceptions with neutral aligned dragons.
Gold dragons associate with silver dragons.
Evil dragons that encounter another type of evil dragon usually fight over territory, whereas good dragons try to resolve there differences peacefully (p 63)