A simple mask is
Red
Green
Blue
These three are the three different textures you can have on a mask and the ONLY colors the mask responds on, any other color and it will go to one of the three that's the closest appearing like that texture in a darker or brighter tone.
If you want more textures there's 1 method I know of, but there's probably more. But in general I'd advice sticking to just 3 since that's the easiest one.
You can get two mask images, name them to two different things. Make them both black, the black works as transparent in this case. You can select 6 different textures, 3 on them each, and draw freely, but keep in mind that wherever you leave it black, it will remain black. Another thing is that if you draw 2 textures over each other that are on Different images it will turn out as glowy.
There's methods on how to prevent this and a fair amount of tutorials in the member made tutorials section if I'm not mistaken, but also on Deviantart. In general though, it's just to copy the mask in
C:\FeralHeart\media\terrains\default
Paste it into your map folder in the terrains folder.
And then draw red, green and blue as you want the textures, and just save it. Shouldn't even be any need to choose save as, but if you must, just save it as a png and it should work fine, can flatten the layers on it to be sure.
