I have recently made 2 different cakes with brown buttercream. Both were eaten the next day but the color turned moss green. Has this happened to any one else? Any advise on this not happening?

what colors did you use to make the brown? I use Hershey syrup and cocoa.

When the three primary colors are blended, you get black. When not using "pure" primary colors, you get grey or brown.

You had a green leftover (yellow and blue). This tells me the red faded, which is very common. Ever since they took Red Dye #2 off of the market 20+ years ago, this happens. Purple flowers (red and blue) will fade to blue often because the red doesnt' hold up in bright light and it fades. When the red disappears, all you have left in yoru purple flowers is the blue.

I used wiltons brown color.