Hey you! I’m very, very sorry for the late reply!
I’ve never taken any (professional) art classes. And that’s maybe why I have trouble with choosing colors as well! But searching on Pinterest helps me a lot. I search for color combinations, color schemes… etc. And about shading, what I do, but there are tons of other ways, is giving the shades (on another layer in photoshop) an extra dark color. After that I change the value of it, so it kinda mixes with the other colors and at the same time it gives a nice shade. With lightening I do the same, but then I color it extra light. Oh yes, with photoshop you can change the color balance in your work so it gets for example a warmer look (youtube has some tutorials). In my opinion the different colors in my work fit more with each other when I change the color balance a bit. Hope this helps!