I am making a wedding cake and it is to be white buttercream with black buttercream decorations. The last time I did a ckae like this, the black icing color ran/faded into the white. How can I keep this from happening?

when I do black on white buttercream I wait to do the piping at the last possible minute. Where I typically do all my decorating on friday for saturday deliveries, if I have black on white cake I will do everything but the piping on friday & pipe on saturday. I also find I have better luck if I purchased a pre-colored black buttercream rather than mixing my own. I use the Satin Ice Black/ Vanilla frosting for black piping, although I do not generally purchase pre made frosting I find I get less bleed with thier black than mine.

Thanks for your response.

When you make your black buttercream start early and get the icing dark gray then let it sit for a while. After an hour if it needs more black add a little more in. The colors darken as they sit and the less color gel you can use on the dark colors the better.

Meringue powder is suppose to help with bleeding. I'd also avoid the fridge and risk of condensation.

I did a white cake with black buttercream piping recently as well. I did a test cake and it did start to become pink underneath after sitting overnight. I decided on doing the piping the morning of the event. Also, start with chocolate icing first, then add the black.