Maybe this extension could help Pixel Stats - Check the total amount of colored pixels you used in your art!
It says that it counts pixels and groups them by color, and also have funtionality for selection. Maybe you could set your grid to 8x8px, then just double click to automatically select an 8x8 area, you could even use a shortcut to move the selection boundaries so you can view different squares and check if there is more than 3 colors