Winsor & Newton’s two watercolour whites have different uses, and your surface can play a role as well.
Within the traditional watercolour technique, the white of the paper is the white used as highlights in a painting, as well as providing a light-reflecting background that contributes to the overall brilliance of watercolours.
However, many artists use Chinese White at the end of their painting for highlights, or to dull some colour mixtures during painting. Introduced by Winsor & Newton in 1834, Chinese White is the most popular white in watercolour. It is semi-opaque and has a blue undertone.
Titanium White is the whitest, most opaque white. It has a high tinting strength and a high covering power.