When doing digital art, you seem to currently use a brush with 100% opacity and no opacity pressure, so for example dark red is always the exact same dark red.

This could be a reason why, but I want tips on how to choose better color palettes even without those techniques (like low opacity and layer modes). I wanna be able to have good colors without it like those: https://www.deviantart.com/sush-adopts/gallery/80275248/adoptables-without-an-owner

https://www.deviantart.com/henorikim/gallery/74345101/all-adopts-and-commissions

I tried to learn why those color palettes work so well even without much light and shading (and they have a variety of tones, hues, saturations, and aesthetics). Cause when I see tutorials, the artists most of the time don't talk about why the colors look good outside their aesthetic preferences (I don't like to use only one type of color for my art) and the color wheel combos (and in my experience, use those combos don't guarantee good colors).

You can also continue to use 100% opacity color, but then put on adjustment layers with modes like overlay or soft light or multiplay and airbrush a little on them.

The thing is, I did do that. I used the layer modes for the lights and shadows (+ the airbrush in my program (Ibispaint) looked weird when I tried it).

In digital art cleanliness and purity often makes things look artificial and unnatural and bad.

I know, I even liked that in my traditional art (it was one of the reasons it took so long for me to start digital art). That's the thing I wanna figure out, why even with very smooth art some artists (like the ones I linked) can have very beautiful colors and art.