Counting the number of specified values that exist in a jitter matrix?

Hello, I have a question that I do not yet have a patch for because I'm not sure where to begin.

I want to count the number of times a specified series of values exist in a matrix, for example the number of pixels that were pure red [255, 0, 0] or the number of pure white [255, 255, 255].

The reason I want to do this is so that I can quantify the number and use it as the basis for sound generation, e.g. lots of blue give lots of a particular sound.

Any ideas? I looked at zl but I'm non the wiser.