That isn't so easy, though. If you're going to try this make sure the gumpaste is totally dry, and that you work with vodka, not water. You want the liquid to evaporate and not sit on the surface of the gumpaste to soften it.
Start with the flower, a clean paintbrush, some vodka and a paper towel.
Take the paintbrush and vodka and slap it on one petal at a time. Make it wet enough that the color starts to lift, then rub it off with the paper towel.
Keep working, removing the color bit by bit.
If you need to get into the crevices, use the paintbrush to wet the area then take it and stuff the paper towel into the space between the petals using the brush.
This is going to take a while, so work slowly and get off as much color as you can while not softening up the petals. Eventually you'll get enough color off that you feel okay with it. Let the flower dry, then re-dust with a lighter color, or leave it the way that it is.