I did a wedding cake for my sisters friends wedding and they requested blue velvet. I made the cake blue. it was quite blue when I covered it, I talked to my sister after the wedding and she said when they cut into it, it was GREEN!!! WHAT HAPPENED???? it was very humid that day and I was worried about the fondant holding up but I never imagined the cake would turn green!! Why would it do that?? 

I know that large quantities of blue coloring will turn your fecal material green... 

but if the cake came out of the oven blue, the only thing I can think of is some chemical reaction occurring to change it to green... but I have no clue what...

A

AI have stopped asking my husband "what color is this fondant to you" when trying to color match. I use to see if what I thought was a match for Tiffany Blue (or any color) was what he thought Tiffany Blue should be. Well, in the Tiffany Blue example he didn't pick Tiffany Blue because he had never heard of it.

My point is one person's blue is another person's green. Until you see a picture of the cake I wouldn't assume something changed or went wrong.

AIt could also have been due to the lighting of the venue.. Soft yellow lights? Might have made blue look a bit green..

did you cut into it before you filled and frosted? yellow eggs and butter can definitely turn your batter green. What it looks like on the crust vs the crumb can be different.