Would it be cheaper to buy chocopan or my own MMF?
You can add melted candy melts or chocolate to your melted marshmallows to get brown...to get dark brown, add a few drops of dark brown coloring to the melted marshmallows.
I'm not sure how much cocopan costs but making MMF is inexpensive.
See if you can find dark chocolate cocoa. I bought mine at McCalls. My fondant turned out almost black. I put in 1/2 cup - should have started with 1/4 cup.