Answer:
So you can have quark anti-quark pairs (charge anti-charge), or add the charges together (red+blue+green = white). This also holds for theorectical particles made of 4 quarks (2 quarks 2 anti-quarks) and 5 quarks (four quarks and 1 anti-quark or the reverse). In your example ud only has two quarks so only two colour charges so cannot exist. If you want a more indepth answer start reading about colour charge.
Explanation:
