It has to be made with Grayscale, to answer you. You can use any art program such as Gimp or Photoshop to make a map, however.
I'm confused with the problem honestly, but I want know it.
The heightmap might have too much detail if you're talking about typing in the heightmap, and it just crashes.
Yes, it HAS to be grayscale. The ground textures, however, don't. They are red, green and blue.
Sorry if I didn't help much. >_<