PPT Slide
A color histogram denotes the joint probabilities of the intensities of the three channels.
h[r,b,g] = N . Prob (R = r, G = g, B = b)
where R, G and B are the three channels and N is the number of pixels in the image.
We can also derive a single variable histogram
where m = r + nR.g + nR.nG.b and nR,nG and nB are the number of bins of Red, Green and Blue.