I think the missing feature is the “Number of unique colors” display. It feels like something we could add to the histogram editor to solve the problem.
It is indeed the thing I use. The rest of the output is just a poor man’s Histogram dialog (can’t be resized, and only gamma+log):
But the histogram alone doesn’t tell everything, the three layers below have the same histogram:
Btw, in your example; 17444 colors in a 6Mpix image is pretty low. Is that a CGI?
PS: plug-in-ccanalyze can be used to return the number of colors in an image, so if this function is transferred to the histogram, some equivalent to this could be kept. And while we are dabbling with statistics, something that isn’t very easy to obtain currently is the average color of the selection. The average as returned by the histogram plugin is the direct average of the values (and we have to do this for each channel). But in a 8bpc image the average color should be the gamma encoding of the average if the linear values (so for instance a checkerboard pattern should return 186 and not 127.5).