There are many answers to this, but one answer might be to take away 6 of the other colors. This is because if 12 marbles are to equal 50%, we need the other side to have the same amount of marbles as the blue. And since the other color marbles add up to 18 and not 12, we need to take away six of the other colored marbles to make it 12.
I hope I helped, and if you need more information, please feel free to tell me! :D
