Use whatever color you wish. Heck buy a black and a white bag of the same weight and mix them together if you want.
Heres a link to an FPS calculator on arnies so you can now figure out what FPS you will achieve with different weights of BBs. You may wish to book mark it, its very handy.
When it comes to BB weight, the heavier the BB the slower its going to travel, however the heavier it is the more resistant its going to be to foliage and wind.