On Saturday someone asked me what gouache was. As I was explaining it’s like watercolour but opaque paint not transparent, they got stuck on “how can paint be transparent when it’s a colour?” It’s not the first time I’ve encountered the misunderstanding that transparent means colourless (like reading glasses) rather than having colour but still allowing what’s beneath or behind it to show through (like sunglasses).
Watercolour is a transparent in that layers of watercolour paint allow what’s beneath it to show through. How much shows varies, depending on the properties of a pigment and how thickly/thinly you’re using it. The more water you’ve added to the paint, the thinner the colour will be and thus the greater the transparency.
Traditional gouache is used with water, like watercolour, but is inherently opaque and matt, covering over what’s been painted underneath. The result has quite a different feel to it: flatter, more solid colours. The exception is what is sold as white watercolour paint, which has opaque properties; it’s often called Chinese white, sometimes titanium white.
It’s the transparent nature of watercolour that enables you to build up rich colours with a sense of depth, layer by layer. To darken tones by applying another layer of the same colour. To ‘mix’ colours on the paper rather than on your palette, such as creating a green by painting blue over yellow or a purple with red over blue. (To put it into artspeak: optical mixing rather than physical mixing.)
The opaque nature of traditional gouache enables you to add detail to a watercolour late in its development, for instance grass in the foreground, or for overworking areas that have gone wrong. Do have a go at a complete painting with gouache alone at least once as it’s a different beast to watercolour. Or “fake it” by adding some white to all your watercolour colours; the colours will be less saturated but it’ll give you a feel for it.