I recently made a birthday cake that was decorated with fondant, I had many different colors and this was the first cake that I have wrapped with fondant decorations. The customer was pleased but I was not too happy with the colors of the fondant. When I first colored the fondant it seemed bright but after I had it on the cake the colors looked dull. So my question is with marshmallow fondant how do you make the colors vibrant or get them to POP?

I am not sure i understand what you mean. I don,t make m/m fondant , i make original fondant, So, is the color deep as you want when you color it or what. Does it look like it gets dull as it sits. Anyway, you can take a paper towel and put a little crisco on it . Rub your cake. then take a clean one and shine it up. Is this what you are asking. There are many sheen airbrush colors on the market and that would be a great way, hth