Rainbow rose

Came across a Japanese ad that goes something like, how about some rainbow roses for Mother's Day? Thought this wacky idea is Japanese, but no, I'm wrong. You won't have trouble finding it outside Japan. Like here or here, etc. I know the roses are white to start with. But... Each pedal seems to contain only one colour. If the split-stem-in-dye method is used, how do they keep the colours from bleeding between pedals?