I’m gonna try to explain it but I really suck at this, anon, I’ll do my best (but this coloring is very simple so I think that’s alright)
First, I suggest you to get one layer per color, it’ll be much easier to add some shadows here and there. You’ll have many layers, yes, but it’ll be clean and simple
Another detail too:
The only actual lines you can find are the details of the faces, I’ve erased all the rest of the lineart
For each layer you’ll need to work on, always check the case “Preserve the opacity”, that way you don’t spill on other layers/color (also don’t forget to lower the blending proportion, if it’s too high, your shadow will blend with the basic color you won’t even see it, the effect wanted here is a concrete and sharp shadow)
I wasn’t daring at all when I chose the colors of the shadows (because I planned to add a very colorful layer over all of this at the end) so I mostly played with the bright/dark option
For more daring shadows, I suggest you to play with the color wheel as well. For example: if the shirt is blue, make the shadow darker and a little greener, the result can be surprising!
after that, I created a new layer above all of this (but just over the characters so it will only influence their colors but will keep the background white)
I like to blend different colors but you can keep one simple color
Here’s all you need for he end: Mode and Opacity of this layer. What comes is really up to you, try everything and see what you like! (On this drawing, I chose the screen option)
The mode of the layer can totally change the atmosphere of your drawing, so don’t be after to try them all :) (and even change the color of your layer! Juggle with warm and cool colors)
Voilà! I hope that’s clear!