Currently reported colors are as follows:
Red (Very common and easy to get. Made with artificing or pulled from dropped/purchased weapons)
Green (Very common and easy to get. Made with artificing or pulled from dropped/purchased weapons)
Blue (Very common and easy to get. Made with artificing or pulled from dropped/purchased weapons)
Yellow (Common. Pulled from dropped/purchased weapons)
Orange (Somewhat common. Pulled from dropped/purchased weapons. Will occasionally drop as loot.)
Purple (Somewhat rare. Pulled from dropped/purchased weapons. Will occasionally drop as loot.)
White/Silver (Incredibly rare. Only from some raid bosses.)
Black (Incredibly rare. Only from some raid bosses.)
This doesn't include the preorder yellow/black crystal