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So how to use? It's very simple. Add a Blender node. Connect your color map to both inputs of the Blender. Connect your tan map to the Blending input. Now depending on which way your tan map goes (white on black or black on white) you darken one of the input channel colors on the Blender. You can also make it a bit more red if you want a coppery tan. Run the Blender to your Diffuse_Color or wherever the color map used to go.

 

Arg... I've been trying this, and running into complications.  I got it all set up like you said, and can sort of get the effect I'm looking for, BUT...

If I use a "black on white" tan map, then darkening Input 1 on the blender node darkens the whole image, and puts a black tanline on the texture.  No good.  Darkening input 2 leaves the bikini area alone, but darkens the rest of the skin... which is pretty close to what I want, BUT... then I'm gonna have to go and darken the rest of the material zones to match the darkened skin.

What I'd like to do is lighten the bikini area and leave the rest of the skin alone.

So I inverted the tan map, and gave that a shot... didn't work, either.  Darkening input 1 now does what darkening input 2 did when I had the original map, it leaves the bikini area alone, but darkens the rest of the skin.  Darkening input 2 now leaves the rest of the skin alone, but darkens the area I wish to lighten up.

Is there a way to invert this, so I can lighten just the areas covered by the map, instead of darkening them, or darkening the rest of the skin?

Sorry for noobish questions!