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There are n=3 choices. For a group of k=3 pixels, there are
n x k=9
possible combinations of colors, {RRR, RRG, RRB, RGG …}.

However if one pixel is red, we are left with k=2 and the following
n x k = 6
combinations are possible, RRR, RRG, RRB, RGG, RGB, RBB.

Therefore there is only a 1/6 chance that the group will be neutral, RGB.