Hmm, well, first things first-
You are using your terrain mask with only Red, Green, and Blue colors, yes?
The black and mixed terrains are caused by any colors other than the ones I specified. However, if your terrain mask uses a lot of fading or super intricate designs, FH might not render it straight away.
Maybe try making a very simple terrain mask first, and see if that works? Make sure the lines between the colors are distinct, and nothing too complex. If FH renders this, then try it with your terrain mask you have now (although it may need to be simplified a bit).
If it still malfunctions, I would suggest uninstalling/reinstalling FH.