Well, if they are dark colours you are likely correct. But here is a suggestion, pipe an icing blob in white underneath where your flowers will be placed, one buttercream blob for each flower. That way the white blob will keep the buttercream coloured flowers from sitting directly on the whipped cream. Make sure you get that cake back in the fridge as soon as possible too. Stabilized whipped cream frosted cakes do not sit out for long periods of time, well. So, if you can, make up your buttercream decorations ahead. Refrigerate them or freeze them. Place blobs on the cake, pop on the refrigerated or frozen buttercream flowers and return the cake to fridge.
Hugs Squirrelly Cakes