The general strategy for masking an effect in matmatic is to do two equations. The first equation describes the math without the effect. The second describes the math with the effect. Then you drive a Blender to run both and choose (or mix) according to the value of the mask.

So:

colorMap = ImageMap("").labelled("Color Map")
color = colorMap

using WHITE initially means it has no effect, but can be changed later in the mat room

generalTint = SimpleColor(WHITE).labelled("Tint")
freckleTint = SimpleColor(ltBeige).labelled("Freckle Tint")
normalSkinColor = generalTint * color
freckles = Spots(1, freckleTint, ...)
freckledSkinColor = freckles * normalSkinColor
freckleMask = ImageMap("").labelled("Freckle Mask")

application of mask to choose between normalSkin and freckledSkin

color = Blend(normalSkin, freckledSkin, freckleMask)
color = color ** gamma
diffuse = Diffuse(color, ...)
 

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