Good paints are really worth it, and a little bit goes a long way, but buying all your paints at once can get expensive. I'd recommend quinacridone rose, hansa yellow medium, and phthalo blue green shade, and phthalo green blue shade. This is the most economical way to get a full gamut of colors. These four colors all work very well together and can make vibrant mixes all around the color wheel. If you need a stop-sign red, just mix the rose with a little yellow. If you need a realistic green, mix yellow and the green with a little rose to make it look more natural.

If I had to add a few more colors they would be neutral tint, ultramarine blue, transparent pyrole orange, new gamboge, burnt sienna, raw sienna, and cerulean chromium.

I have more info, and links to recommended materials here. (There's no ads, and I'm not making money off of anything.) https://connor-johnson.com/2017/06/07/getting-started-with-w...