That is not a color image - it's an indexed image - so there are not 3 color channels. What do you want? Do you want a histogram where you can control the color of every bar?
By the way, a true color image would either have 3 1-D histograms (one from each color channel), OR would be a 3-D histogram (scatterplot) showing the full color gamut, where the XYZ axes are R, G, and B and there is a dot/marker at every (R,G,B) location in 3-D space.
