How would I count the number of cells of a certain color using grids?

I am playing around with some pixel art. 

I am using the grid system to design my image, like the mushroom above. Once the image is done, or in the process of making it, I would like to have a tally at the end that says 24 white; 45 red; 18 black (or whatever the correct number is).

Any suggestions on how to do that?