To add or change skin color texture make a texture with needed elements. The image below can used as an example.
It is important for texture to be in gray color because it has soft blending. Colors will become darken or lighten depending on the blend color. The effect is similar to shining a diffused spotlight on the image. If the blend color (light source) is lighter than 50% gray, the image is lightened as if it were dodged. If the blend color is darker than 50% gray, the image is darkened as if it were burned in. Painting with pure black or white produces a distinctly darker or lighter area, but does not result in pure black or white.
Add or edit the softlight_tex parameter with the texture you want to use.
See the example of effect API usage.