@tingchen00 I really just apply how I color in Photoshop to a new program, and look up tutorials to look up where certain functions are. I suspect our color methods are super different because I rarely use eyedropper but I can give an abbreviated look into my process in case it can help?
1.
Base layer: On their own layers, I fill in the base colors of everyone's skin and clothing (I made seperate layers for each character's skin and articles of clothing). I also added textures to the base layers (ex. On her jeans, I think I used a really big rough pencil or blurred edge watercolor default brush in a light color on a low opacity (20-30%).
2.Shading layer on top of base layers: layer properties set to Multiply at 80% Opacity: On top of the base layers, I add my shading layer. Multiply allows the base colors to interact with the hue I'm using to shade (Red, in this case). With these settings, I roughly block in the basic shadows with a large g-pen brush. From here on out, shading is just a process of erasing and painting back in with an assortment of default brushes at varying opacities. I used a lot of watercolor brushes but honestly, it is mainly through experimentation that I achieved results so I suggest a lot of fudging around with different settings.
3.
Put shading layer on opacity lock and apply a gradient in a brighter red: Finished piece and a closeup. This is just an after effect, but I put the shading layer on opacity lock after I was finished with it to apply a gradient. I suppose you can also select specific areas to alter the color, as well.
...So this is really vague and terrible, haha. But if you have specific questions about functions or anything else that I horribly described above, please ask.
(I'm hoping this sticks because I accidentally deleted this reply the first time, lol)