I'm planning on making a cake frosted in white buttercream with black buttercream decorations. I'm afraid the color from the black is going to stain the white or leak into the white. Does that ever happen? Would refrigerating it help it, or hurt it, or make any difference in helping the black not leak onto the white?

I'm pretty new at this, but just from my personal experience putting it into the refrigerator then taking it out will probably make the colors bleed into each other. If nothing needs refrigeration (the fillings or frosting) I would leave it out. If you have time maybe a trial run? Test both in and out of the fridge. Good luck!!

This is just something I've tried with dark colors on white, I put a good heaping spoonful of meringue powder. It makes the buttercream crust quickly to avoid the dark color from bleeding onto the rest of the cake. You might want to give this a practice run.

I use a crusting buttercream and have never had a problem with dark color bleeding onto white.

I've always used IMBC, which doesn't crust, so maybe that's what my problem was.

As others have said a crusting buttercream will help solve the problem. I've also found for me, that letting the cake icing crust before starting to pipe helps.