MY mmf is cracking. I made gray and navy from the same batch and the gray is fine, but the navy is cracking. What am I doing wrong?

How much food colouring did you put into the navy? If fondant has a lot of colouring the texture is affected. try kneading some glycerine in if you have it. If not knead some shortening in.

could the fondant be drying out??
I recently bought a video by Jennifer Dontz.... ' how to cover a cake in fondant" it has a recipe for adding white chocolate to store bought fondant.... since I bought the dvd at my local cake store, I haven't had any problems with fondant drying out or getting the "elephant skin" look!!
Before I bought the dvd, I had NO confidence in my own skills covering a cake in fondant.... and since I bought it, I have succesfully covered many small cakes, AND a 15, 12, 9 and 6 inch wedding cake!! Even I was impressed with my own skills, and that NEVER happens!!
If you have access to a good cake supply store, I highly advise getting the DVD.... if you don't have a good cake store, you can probably find it on line,
try this web sight :
www.countrykitchensa.com
good luck! cindy