[kamocat]

I chose the saturation plane because it provided even shading for all three squares. The intensity plane had each square at a different darkness (and actually varied at the edges of the squares), and the hue plane would not apply (all three squares are wildly different hues, and the hue of white is unpredictable).

If I was looking specifically for circles, I would use the circle finder. In this case it's a very clean image, and so analyzing all the particles (without even an erode and dilate) will produce excellent results.

If it had been a real image, I probably would do some more processing to get a clean result.