Consider a 4 X 4 block on the board. If it contains (more than) three green-colored squares, it will violate the condition, since we will be able to place a triomino on it in such a way as to be all green. So no 4 x 4 block can contain more than two green-colored squares. There are 16 such 4 x 4 blocks, meaning that no more than 32 squares can be colored green. That this number is best-possible can be seen by looking at a standard green and black checkerboard.
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