I have a client requesting a bright blue cake batter for this Saturday. I only have two tested flavor a for stacking--butter yellow and rich chocolate. I am concerned that the cake will turn out green with blue food coloring or it will need so much coloring it will be bitter. Any advice is welcome on how to achieve this look!

Might work better with a white cake. There are recipes for white cake in the recipe section. ...

I have found butter cakes with added food color turn out slightly darker than the color you thought you were going to get. But never green.  So the color will not be spot on, but it won't be green. Make sure to use a good gel food coloring, not a liquid one. You could always whip up a 1/4 batch of your cake recipe tonight just to test the color. I bet the would help you determine any adjustments that need to be made.