By the book, but just for now...
For my current campaign, I pretty much stick with the traditional chromatic/gemstone/metallic mixes of dragons, more or less for ease of use for now. I was considering to trimming down the # of dragons to 6--basically using the basic 6 dragons introduced in the Basic D&D boxed set (white[N]/black[C]/green[C]/blue[N]/red[C]/gold[L]), & even going w/ the L/N/C alignments they had in that game, though changing around the G/N/E AL... But, I still wanted to include old favorites like the silver, amethyst, & copper somehow.
However, I still want to reduce the number of dragons in game, & I've worked on a plot/setting idea for this:
The sheer number/variety of dragons are a result of times when dragons were dominant and plentiful--days which are long over. Dragons are pressed for resources by the mortal races & other dragons, & their numbers dwindle rapidly. However, rather than facing utter extinction, the dragons are "evolving" along another path: many are interbreeding. Species which would never have mixed normally are pairing off, creating offspring that shares in both of their parents' traits/legacies.
For the most part, many of these pairings are of dragons of similar color: emerald & green dragons; topaz, brass, yellow, & gold dragons; sapphire and blue dragons; crystal & white dragons; black & shadow dragons; orange & copper dragons; etc.
The "new" breeds of these unions are referred to in simple terms of color: shadow/black dragon mixes are reffered to as black dragons; emerald & green dragons are referred to as green dragons; etc. However, their abilities & their personalities are a unique mix of their parents.
Stepping away from the story element & into a mechanics-based perspective, I'm basically creating 1 dragon species of each color: red, blue, yellow/gold, green, black, white, silver/grey, orange/copper, brown/bronze, & purple. Alignments tend to lean towards a certain mix, but anything's possible. Despite the mix, all of these new dragons have 1 breath weapon/immunity. Appearance is a mix of the parent dragons (so not all dragons of a particular color look the same).
Then again, I'm considering reducing the # of dragons even further, and basing each dragon species around a particular element/energy/magic: fire, earth/acid, air/electricity, water/cold, sonic/thunder, force. Color would most certainly not be fixed, though it'd be within particular ranges (fire dragons being reds, oranges, golds, browns; etc.). I'm still considering it, but I'm up in the air about it.