I've been having issues recently with my colors from rolled buttercream bleeding. Red into white, purple into white. It has happened with icing that was recently colored and also with icing that was colored a week prior. What is the story?
I've got 40 wedding cake cookies that have bleed purple into the white and I can't stand to think of starting over!
I'm sorry for your plight, but there's not a lot you can do. I don't think it has as much to do with how long ago you tinted your buttercream but with how long the two colors have been in contact. When there's a really dark color against white, you almost have to do them just hours before you need them - not that there's not a lot of advance work you can't do days before and just put the white and purple in contact at the last minute. Here's a suggestion that MIGHT work: the other day I was drawing with a black food pen and went really astray. I was able to cover up the black with that white coloring that Americolor sells (Wilton has it, too). You might try using this to paint over the white part on a couple of cookies to see if it works, but if it does, save the painting for the last possible minute on the rest of them, because the purple will probably bleed through that after a while, too. Hope this works or you can find something else that does!
Meringue powder added to buttercream is suppose to help stop colors from bleeding together as fast. Not sure how it would work with the rolled stuff.
I know when doing royal icing, if you let the colors dry between colors that helps with the bleeding. Maybe you can do the purple parts and let them sit for a while or over night, then add the white?