Channel packing

Channel packing is a way of combining grayscale masks or textures into a single image by placing them within the RGB channels of a color image.

First, create the individual textures. For example, in a tint mask for the Nolthrirf arms, create a simple black and white mask of the skin in the character texture.

Once you have created your three desired textures, make sure your image mode is set to grayscale. (Image>Mode in Gimp) Next, compose the layers into a map (Colors>Compose in Gimp)

Choose RGB for 3 textures and assign the textures you made to the correct (according to the standards on the materials page) channels (R, G, and B).

This will create the packed texture.