i think i remember reading some where that you weren't supposed to use royal icing the same bags you've used BC icing in. is that true?

the reason for that is that the grease from the buttercream will break down the royal icing. I always use the plastic throw away bags (too lazy to clean them) you could use the plastic just for Royal or have one or two new pastry bags marked ROYAL so you only use those with royal icing.

HTH
tara