A very simple, yet very effective, method of determining if a pixel is in an area of uniform color is to take a block (perhaps 20x20 pixels) that is centered at

A very simple, yet very effective, method of determining if a pixel is in an area of uniform color is to take a block (perhaps 20x20 pixels) that is centered at the pixel you are testing and see if that pixel is within a certain standard deviation of all the pixels in the block. Adjusting these two "knobs" (size of the sample area, and standard deviation threshold) will let you fine tune to your needs. This should highlight edge regions too.