Spray paint that is meant for glass. But it works really well on fabric for a translucent, shiny look too.
Buy some white silk/faux roses and some spray paint meant for glass. Spray on top of the bunch. Not too close or the paint will drip. I think this would look really neat to use different bases flowers with contrasting paint, such as orange flowers with red paint, or purple flowers with blue paint. You could also sprinkle some glitter over the tops while the paint is still wet!