We have 8 A's, 9 B's and 8 C's.
If you notice that each trimino covers exactly one A, one B and one C, you can see immediately that you have to omit a square colored by color B.
Now, we can repeat the same reasoning with a different coloring (obtained using vertical symmetry):
There's only one square which has the color B in both colorings.
You can easily find more similar problems, if you're interested in them, if you google for keywords like invariant, domino, chessboard