I have used the Wilton Brown and the Ameri Color Chocolate Brown gel food colorings.. I made a cake that was a beach theme and I wrote in the sand (brown sugar with brown icing underneath it) Happy Birthday.. I took pictures and it looked great. The next morning the Happy Birthday was a dark icky green color! So I wrote it again and it mixed up the icing and took off the icky layer. This is not the first time this has happened and I must say, I'm afraid to use brown! If I hand the cake to the customer and they don't eat it til the next day.. I dont want it nasty green!

Anyone know why?

Would brown powder food coloring work better?

I don't know specifically about food color stability, but in color mixology (paints, etc.) brown color is made by mixing green (blue and yellow) with red. My guess is that some of the red is breaking down and leaving things a muddy green.

How about using chocolate buttercream instead?

Since Red Dye #2 was removed from the market in the '70's, reds have not been stable and it fades in bright sunlight, under florescent lighting, etc. When the red fades, you're left with the remaining two primary colors, blue and yellow, which makes green.