Color theory helps a lot, with realism you'll not only have to know to make specific colors, but be able too properly contrast those colors in different lights with black and white. Here's a simple explanation:
https://colormatters.com/color-and-design/basic-color-theoryBest way to do get started if you want to get right into painting imo, is painting flowers. They have a good monocratic range of colors in their pallet, plus their fairly easy to master if you give it some time. You'd be surprised how much you can achieve with just red, blue, yellow, black and white.
Getting to know how to create realistic images does take time. If learning about light and shading is your goal, invest in some gray paper and some white and black (or blue or whatever) charcoal. The gray paper is a good middle ground for you to work with light and dark. It can be a little confusing for some, but as long as you sketch out what you want in pencil and fill in the light (white) and dark (black/blue/etc) you should be good.
Hope this helped and remember to have fun with it!