I tried mixing MMF brown and it started crumbling up and falling apart does anyone know what causes this? Does the corn starch dry it out sometimes once you sprinkle it to keep it from sticking to the surface?

What kind of brown colouring were you using? Mix in a bit of crisco, that sometimes helps, and keeps it from sticking.

try subbing some of the PS for cocoa powder. Then to get it darker use brown gel colors.