So here is the basic way to use the Spots. Add a Blender between your color map and the PoserSurface. Connect both Input_1 and Input_2 to your color map.

Connect Blending to your Spots.

Set the Input_2 color to RGB 235, 222, 173. (A pale yellow.) You can play with this color later, but try mine for now. What this is doing is multiplying the color map with that color. When you multiply a color map with a single color like this, it effectively stains it, just like a wood stain on wood. 

Now because the Blender is following instructions from the Spots, there are places where the color map is multiplied with white, and others multiplied (stained) with the pale yellow, and still others that are blended between the two. So we get a pattern of staining that follows the pattern of the Spots. FRECKLES!

The reason this works is because numerically white is the same as one (1.0) - and if you remember any math at all, you'll remember that x times 1 is just x. So staining white makes no change.

Try different colors to get used to what happens with different stains.

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