Just to illustrate, consider these Siegel images from the same page of the same catalog number.
We have no idea how these images were generated, if any 'post scanning filters' were applied or even if they were scanned with the scanner lid open or closed. Virtually useless as any kind of color ID.
Below is the same postal card, using the same scanner, the same scanner configuration, no filters... but look at the difference in color between scanner lid open and scanner lid closed. (Scanners are calibrated from the factory using scanner lid closed, unless you recalibrate you scanner, always scan with lid closed.)
As Germania mentioned, a single scan showing all stamps side-by-side is about the best you can do. It will show the delta between the stamps. Of course it will not tell us what color they are, each of us will see different colors depending upon our graphics chipset in the computer, the LCD screen manufacturer, graphics driver software, and ambient lighting.