Masstone and undertones in watercolour, what are they? Masstone is the colour as it comes straight out of the tube; undertone is how the colour looks once it has been diluted with water. We open up these Professional Artists’ watercolours to reveal the hidden undertones of Transparent Yellow, Quinacridone Gold, Green Gold, Phthalo Turquoise, Prussian Blue, Hooker's Green and Payne's Grey. Some undertones are quite different from the masstone. For example, Green Gold is a dark olive colour straight out of the tube, but it becomes a bright yellow-green when diluted with water to reveal the undertone. It is worth considering the properties of both the mass and undertones when choosing a colour.