Okay, /sci/, I need your help, but this may seem like a very specific question. I'm an engineer who's starting to get interested in physics. Right now, I'm reading about the four basic interactions and there's some stuff I don't get.

So we have hadrons, which are formed by quarks. These quarks can be green, red or blue acording to quantum chromodynamics. There's a rule that says that in every interaction, the 'sum' of the three quarks must be red+blue+green.
Does the same thing apply to mesons, which are composed by only two quarks?. How can the rule be applied (if it indeed does apply) if there are only two quarks? Are they multi-colored quarks?