1. Fire up Photoshop/GIMP/etc—anything that can handle layers.
2. Open the default skin you use for your model.
3. Add a layer, because we don’t want to mess with the original layer.
4. Use magic wand (with sample all layers enabled), click on non-skin area (background white, for example), reverse selection so only skin areas are there.
5a. Inside that area, paint with a white paint brush the spots that should show paleness; use layer properties to set the layer to “lighten” mode and drop opacity down to 20% or less.
5b. Alternatively, pour in some brown, set layer to “multiply”, drop opacity to 20% or less; use eraser to scrub your desired tan lines.
6a. No matter what you do—5a or 5b—turn off the background layer with actual skin and save the uppermost layer as transparent .png.
7a. In DAZ Studio, go to Surfaces tab, pick the bodily diffuse slot, then the pulldown menu in the corner—open LIE, adjust settings as indicated (image file size), load the skin you’re messing with, place this new transparent .png over it, and use the resulting file.
6b. Or save it as a full skin, and load it to DAZ Studio as diffuse instead of the one you’re using now. Remember that if you brown up your characters, you have to retouch the face and other bits too.
More complicated, sure, but then you know what you’re getting.