First of all gouache is pronounced “gwash”. It doesn’t exactly roll of the tongue, and I think it sounds even worse in a Northern Irish accent, but there you have it!
Watercolour and gouache look like they’re pretty much identical mediums, but the biggest difference between them is that gouache is a lot more opaque due to the addition of chalk in its make up. When you put watercolour down on paper, any outlines that you’ve drawn will show through, whereas with gouache, they won’t show up just as clearly.
White designer gouache is usually used alongside watercolour to create highlights on watercolour paintings due to their lovely opacity.
Gouache will allow you to paint on toned paper AND it's easier to cover up little mistakes thanks to its opacity! YAY!
Watercolour has a luminous quality due to its transparency. This happens because light is able to travel through the pigment and reflect of the paper. Therefore gouache will have a matte finish because it’s opaque. Still with me?
One quality that both paints share is that they can both be altered after they dry by just adding water. This is down to the pigment and water-soluble binder that is found in both mediums.
Illustrators tend to prefer gouache over watercolour because:
1. It dries quickly
2. You can use it to create solid blocks of colour and can be used for intricate details. This differs to watercolour as it dries much slower and is not as controllable.
You can use both paints together though- gouache can be used with watercolour and vice versa when painting layers, or they can be mixed to produce different levels of opacity.