What in the world does gouache mean? Pronounced gwash. It’s actually pretty simple. If we say watercolor we usually mean water based paint that is transparent. If you put one layer on top of another, you can see both. Gouache is watercolor paint too, but it is opaque. You cannot see what is under the top layer. Usually. Gouache is often used for illustrations in advertisements and magazines. And when it is used in paintings it looks very different from transparent watercolor. It also is useful in transparent watercolor paintings. If I need to use white or black, it is gouache. In the case of white, I try to use the white paper with no paint, but sometimes that is not possible. In the case of black, I try not to use it because it tends to look dirty compared to transparent paint. I think the most useful black is a mix of indigo and sepia. Also sometimes gouache can be used to cover a mistake. But I would never admit there was a mistake in the first place.