I used gouache all the time before I went digital. My entire comic book illustration career contained lots of gouache in the cover paintings I did. I would use it along with watercolor, acrylic, pastel, colored pencil, markers, ink, etc.
The most important things about gouache you need to know are:
It’s really just watercolor with additional inert chalk pigment to increase the opacity, and the concentration of pigment is higher.
When gouache dries, it will be a bit lighter in value than when it’s still wet. When you paint, you need to know that, especially when trying to match colors or continue working on an unfinished area.
The best way to paint gouache on areas already with dried gouache, is to lay on the paint as lightly, decisively, and as quickly as possible, so you don’t disturb the paint that is already dried on the paper and wet them again and pick them up with your brush, thus taint the wet paint on the brush.
Allow each layer to dry before applying the next layer, or they will mix and not in a good way. You do not use any kind of wash/glaze with gouache once you already have a layer of paint on the paper. It’s best to treat each layer of paint as opaque, separate layers that overrides the layer underneath almost completely. Do not try to use it like you would watercolor or any other transparent medium.
Syd Mead is a master at gouache, since that was his main medium for his body of work.
If you want less trouble, with each layer of paint drying quickly and won’t be disturbed again, acrylic will be a lot easier to use than gouache, but it isn’t as delicate in its handling for detail as gouache is. If you really don’t care about quick drying and just want the most satisfying medium to paint with, nothing comes close to oils (that’s why it remains the most beloved traditional painting medium still), and today we have quick drying oils as well as water-based oil paints that handles pretty much identical to traditional oil.
BTW, you do not need to learn color theory through traditional paint mixing. Color theory is independent of the medium you use. How paint mixes (as well as how digital colors mix in RGB) are separate issues from general color theory. It’s more important to learn general color theory than idiosyncratic paint/color mixing systems, because it’s the aesthetic, scientific, and psychological elements of color perception that truly matter, while technical aspects can change from traditional to digital or even just among different traditional mediums.
