White pigments show almost no drying shift in watercolors. Zinc oxide is a slightly warm shade of white and completely absorbs ultraviolet radiation at wavelengths below 370nm. The pigment itself is extremely cheap, and when watercolor manufacturers offer a single white paint, this is typically the pigment they choose. It covers reasonably well and mixes smoothly as a bodycolor with other paints. Titanium white PW6 is a completely lightfast, semiopaque, lightly staining white pigment, available from about 30 manufacturers worldwide.
White pigments show almost no drying shift in watercolors.
For a detailed explanation of these ratings see What the Ratings Mean .