Just a thought - could you build up some sort of reference chart for basic values, by sampling say, skin tones. Then use the RGB numbers to check against ?
If that is feasible - you could then move into the more difficult areas and develop your own checklist of key values.
It is hard to visualize your problem, but I know it is much more common than many people realize.
I hope you get some first hand feedback - it is an interesting problem.
Best of luck.