The light reflecting off the white paper is an integral part of water colour painting, yielding watercolours their enriched translucent quality. Leave areas of white in your painting to create that sparkling feature, by painting around them. Therefore, you must be focussed in your attention to detail. However, not all highlights are pure white and therefore during the early stages of development, you will have to decide whether to reserve areas of a pale wash or build up very dark tones.
Masking fluid is one of the aids to achieve white highlights but should be used with extreme caution.