I started this topic once before.
I got some very wise replies, but none that I could implement.

Here's some background. Start with a digital image of a subject that SHOULD be
against a nearly white background.  However, the background is actually
somewhat gray or a very light yellow to light red.

Sampling the current color of subject is easy enough.
But, how do I compensate for the color of the background?

A straight subtract of the colors doesn't come close.
The goal is digital color comparison of several subjects.
 
Is there a formula for color compensation?
Should I use a 3 component system or a 4 component system?

Any good color resource references?

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Pierre A. Zippi
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Home email: paz@...
Home web: http://web2.airmail.net/paz