Not sure if you're still looking for a good white highlight, but I like to use a Chinese White watercolor with very little water (just barely enough to wet the brush). The one I've been using most recently is the Windsor & Newton Cotman watercolors.
Another reason I like it is because you can very the opacity with the amount of water you use, so if you want a completely opaque line, you can have it, but if you just want to lighten up an area, you can do that too with the same tool.