I have to be honest that most of the time I don't actually know what I'm doing and that I have no idea how most of my pieces are gonna turn out. My work process is usually based on "Fuck around and find out", haha. I'm happy to know that it apparently doesn't come across that way, though.
A lot of it comes very naturally to me simply because I've been drawing non-stop for so long, but I can give you some small tips that really help me:
1. Have as many references as possible!
Here's what my reference sheet looked like for the Jayvik piece:
It helped me a lot to understand the overall color scheme I wanted to convey. Lots of very cold tones, pinks and very light blues and greens. These colours sorround Jayce and Viktor throughout all of season 2 and I wanted to keep them, especially since in my piece they are lying in the glowing hexcore.
Don't shy away from using references, get as many as you possibly can! Look at other poeple's art too and try to understand how they work with colours.
2. Work with complementary colours!
Since I paint a lot of romantic illustrations I want them to look pleasing and comforting, which I can accomplish by using complementary colours! You see this a lot with couples that are blue and red coded, for example. And I wanted to do the same thing in the Jayvik piece! For that I used the highlights in their hair!
Viktor's highlights are a soft pink hue.
While Jayce's are a soft blue hue.
The colour wheel works perfect for figuring out if two colors compliment each other because they are literally right across from one another!
3. It doesn't have to be true to life.
Pretty self-explanatory, but I thought I'd add it in here anyways. It's important to understand how colour and light works, but you don't always have to follow the rules. Does the rim light look cool but it makes zero sense? Who cares! Keep the cool rim light! Just have fun and fuck around.
4. A little trick to make your life easier!
I'm not excatly the best at colour theory, I still struggle with it quite a bit, but here's a little trick I like to use from time to time:
If you want all your colours to look coherent, take one specific color as your flat colour. Choose a hue that you would like your piece to have. Like this:
Now you choose whatever colours your characters have and paint them in. For example, here are the skin colours I chose for Jayce and Viktor:
Looks off, right? These colours don't fit the overall piece at all. So what do we do?
Turn down the opacity! It's that easy, wahoo!
I went from 100 Opacity to 72 for this specific illustration. And look at that!
It's so much nicer already! Now you know what colours to use as your actual flats! Just repeat this with every other part of your illustration and you'll have a great starting point. :)
I really hope this was helpful! I'm not an actual teacher and I don't have a proper illustration degree, so some things might not be completely accurate, but I thought I'd try my hand at this anyways!