White gouache can be a fantastic tool for adding final highlights to sketches. It can make cat whiskers, snow, or sailboats pop right off the page, and I especially love working with it on toned paper.
Since watercolor and gouache are both water-based paints, you can mix them to create more opaque pastels. I like to add them to earth-tones for a range of sandstone and beach hues.
I've recently started experimenting with a white gel pen, too. It's perfect for creating really fine details, like the ribs on a very small feather. Drawing in those final brights feels a bit magical!