JimP
Hmm. 1, 2, and 3 are red, orange and yellow. Adding white to red produces orange, and adding white to orange creates yellow. So, no mistake there. 4, 5 and 6 are technically out of order if you're ascending color via adding white, but the 3 numbers represent a form of commonality.. I can't quite make out the color of the 8. Is it purple or black? Anyway, each color is an amalgam based on the color preceding it, except for the number 9. So, 9 is what I would call "the mistake" in this image.