Gouache: how it differs from watercolour and when to use it

Gouache is a water-based paint with the same binder as watercolour, but with more pigment and an addition of white or chalk. That single difference in the recipe changes everything: gouache does not glow through, it covers whatever lies beneath it.

Light over dark

In watercolour the light comes from the white of the paper, so the brightest areas have to be planned and left untouched from the first stroke. Gouache works the other way round: pale colour goes over dark at the end. That is why it can be used on coloured or grey paper, with the highlights added once it is clear where they are needed.

Why botanical illustration favoured it

Gouache dries to a matt, even surface with no streaks and no sheen, it renders flat areas of colour faithfully and it reproduces well. For centuries it therefore accompanied drawings of plants and birds prepared for printing, and it still suits work where clean, uniform colour and a firm outline matter most.

Two things that surprise beginners

1. The colour shifts as it dries. Dark tones lighten, pale ones darken. Matching a tone while it is wet almost always misses, so it helps to keep a sheet of dried samples beside the palette.
2. The layer underneath dissolves. A brush carrying too much water will lift dried gouache and blend the two colours. Further layers go on quickly, with thick paint, without returning to the same spot.

Together with watercolour

The two paints combine happily on one sheet: transparent watercolour builds the background and the shadow, while a few touches of gouache bring back the light that watercolour has no way of recovering. Only the proportion needs watching, because gouache used everywhere takes away the transparency that is the greatest asset of paper.

Works on paper that rely on exactly that transparency are in the watercolours gallery.