That is a question I've been wondering myself. I know in some ads I see "This ghost is compatible with all major lines" but I don't know if that is for all ghost varieties or just the one (green, butterscotch, etc.)

If I had to guess I would say a green ghost bred to an orange ghost would produce ghosts, but I don't know what kind. Is the green, orange, etc. part proven genetic or just attained through selective breeding?