Main window of this functions looks like this:


It's divided in two imaginary parts: display and settings. In first deal you can see, how many times is active R, G and B color intensity used, in second part you can set type of display. If you wan't to know exactly how many times are colors used in image, push 'Color usage' button.
In dialog, that will appear is list of used colors placed. Number of colors included is limited by setting 'Minimal count of color for one item' in tab 'Miscellanous'. If image contains many colors and you have a slow computer, you should select a big number (500 or so), otherwise for the same image will be 10 probably enought (all depends on computer speed).