Make eyedropper tool true to Diffuse colors

This is an urgent problem with the Eyedropper tool in Texture Painting that would make life a lot easier if it implemented.....

In the example picture I'm using a "base" unified color texture combined with a "extra" texture to add details while keeping it easy to correct mistakes.

When using the Eyedropper in the "extra" layer while keeping "sample merged" off, I can only sample the orange color, but not any mixed hues, since the picker is told to only look at the colors on the active layer. And that is fine.

But.... what if I'd like to pick a hue that results from the mixed colors of the two layers? That's what "Sample merged" is for right? The problem is, that the tool will also pick up the influence of any lights or HDR in the scene, making it impossible to pick the mixed colors of different layers without lighting throwing off the shade!

Picking a color that is true to the mix is really important when painting skin, and painting in the "diffuse color" render pass is not a viable alternative because it makes the object a unified colored mass, thereby hiding any hint where the nose of a character etc would be.

If there is any way to set the color picker to ignore lights I apologize, but I haven't managed to find it yet