How can I understand color?

I've been drawing since I'm 6 and got pretty decent at it, but I never colored or even shaded anything. I'm 20 now, trying to learn how to paint with gouache, tried colored pencils before, but everything looks child-like, the colors don't match what I want, the line strokes are visible, just absolute trash. I just can't understand "filling" space, only lines, but I want to