Apple Color 1.0 - Manual (Page 156)

156 Chapter 8 Video Scopes
For example, images with a red color cast either have a significantly stronger red
histogram, or conversely will have weaker green and blue histograms. In the following
example, the red cast in the highlights can be seen clearly.
R, G, and B
The R, G, and B histograms are simply isolated versions of each channel’s histogram
graph.
Luma
The Luma histogram shows you the relative strength of all luminance values in the
video frame, from black to super-white. The height of the graph at each step on the
scale represents the number of pixels in the image at that percentage of luminance,
relative to all the other values. For example, if you have an image with few highlights,
you would expect to see a large cluster of values in the Histogram display around the
midtones.