General

@xronellx Color theory itself is pretty simple, it's basically just color combinations, primary/secondary/tertiary colors, complementary and analogous colors, etc. Essentially a bunch of vocab.

I guess I didn't really explain myself well but what I really meant by saying "color theory" is... the fundamentals of how-to-color-stuff-so-it-looks-legit. This includes color theory but also a bunch of other concepts like light and shadow, how surroundings affect colors, texture, temperature, saturation/intensity, etc. The thing with all these various concepts is that they're all individually easy to learn, but creating a nice product requires using all of these various things in tandem with each other. And that is much more difficult and can only really be acquired with time and practice.

@goodfortuna: Is there an efficient way to learn color theory?

Is your issue with understanding color theory? Or coloring techniques for a certain medium?

Anyway if you want to go traditional it's going to require some investment. True for any art medium anyway. I would recommend watercolor as it's one of the most popular mediums so there are more tutorials and examples out there. You can't get as high color saturation for watercolor and it's impossible to "erase" mistakes but it's portable, easy to set up and clean, and less expensive than other mediums. Make sure you use at least 90 lb paper though. Buy a watercolor or mixed media sketchbook from an art store. And try to buy tube watercolors. Dried watercolor paint is a pain in the pooper to get better color intensity out of. Especially yellow, yellow paints in general are weak af.

For a sec I thought you were talking about another sort of "adult" colour book. If all you're doing is practising shading and colouring I'd say a book would be nice and fast. However, you'd probably find more inspiration in colouring in your own work. You can either invest in some good marker now or buy some watercolour paper for short term temp practise.