Hi, THe morning after I finished my cake, I found little light green specks on my brown buttercream frosting.  I left the cake out overnight in my air conditioned home like I usually do, so I'm sure it didnt get too hot to start molding.  I used brown gel to color it, because I didn't want chocolate buttercream. Was there some kind of reaction with the brown color? Help!

It sounds like you didn’t mix the color in well enough   Did you start w/white icing or  color it something like orange 1st?  And what brand of color did you use?   

idk -- sometimes salt can speckle icing

Oh yeah K8 I forgot about that.   Especially as she they are small.  Either way it probably means it wasn't mixed well enough.