In ArtRage 2.2 we added the ability to sample paint from a single layer (prior to blending or adding lighting effects).
Choose you colour sampling tool, and look for the control in the tool options window Single Layer'. That will just take colour from the current layer.
Also ensure 'With Lighting' is off. When it's on you're sampling the end-result pixels from the screen after lights and metalics have been added.
In the example pic the multiply function of the top layer makes the paint dark. If 'Single Layer' is OFF, the sampled colour in the middle would be dark. If 'Single Layer' is ON the sampled colour would the the original yellow.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"