Adding white to transparent watercolour will make it more opaque, essentially transforming it from aquarelle into gouache. Physically, so-called 'white' paint consists of tiny, transparent flakes, which diffuse or scatter the light passing through them (rather like water droplets in the form of clouds or snow). When adding 'white' to a hue, we are essentially increasing the scattering of light within the paint layer; hence more white light (from the light source) gets reflected and the 'colour' looks paler and increases in opacity.

Whenever you add white to watercolour it will vastly decrease the purity of the light reflected, and inevitably cause a chalky appearance in the paint. White will always cool and dull colours, making the normally transparent far more opaque. This need not be a bad thing, as I have mentioned elsewhere.

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