If you didn’t save the whites in your painting and need some really bright highlights, you can still lift the paint. Lifting means erasing (lifting off) paint after it has dried on the paper surface. To lift watercolor I use a hard scrub brush called the eradicator from Rosemary and Co, but any flat brush will work.
To lift paint, use a damp brush, and gently rub the area you want to lift with the edge of your brush. Keep a paper towel handy and alternate between rubbing the brush on the spot where you want to remove the paint, and blotting it up with the paper towel. For best results, rub the brush in only one direction.
It’s important to keep dabbing with the paper towel as you’re lifting, otherwise you’ll end up just pushing paint and dirty water around, and the paper will become too wet. If the paper becomes too wet, the surrounding paint will start to bleed and bloom.