Well, consider the unluckiest case: In whatever order, you manage to remove nine of the red marbles, nine of the blue ones, nine of the green and all of the yellow and orange mix. So far, you've removed 37 of the marbles (9+9+9+10 = 37), yet you don't have 10 of any single color. If you remove just one more marble, it will be red, blue, or green, and you will have 10 of the same color. So the answer is 38 marbles. :)
