The brightest white is the paper
The cleanest method is to decide on highlights before painting and leave those areas untouched. A light drawing and simple plan are enough to preserve reflections, highlights and breathing room.
Reserves and masking fluid
For small details, use masking fluid on fully dry paper and remove it when the paint is dry too. Do not leave it for too long or rub: watercolor paper responds best to gentle handling.
When to use opaque white
Chinese White or white gouache are useful for restoring a spark at the end, softening a texture or correcting a detail. Apply them sparingly: when they cover large areas, watercolor’s characteristic transparency is lost.
Plan highlights
Before wetting the paper, lightly mark the areas that must remain white. In flowers, glass or water, a few well-placed reserves make light read immediately.
Protect the paper
If you use tape or masking fluid, test them first on a scrap of the same paper. Remove them slowly and never over a still-wet area to avoid lifting fibres.
Recover a highlight
When an area has become too dark, dampen it with a clean brush, wait a few seconds and lift pigment with absorbent paper. Repeat patiently instead of rubbing.
Work with contrast
White appears brighter when surrounded by slightly darker values. Sometimes you do not need to add white: gently strengthening the nearby shadow is enough.