I have this brush on Firealpaca called “Flat Brush”
which, if you increase the rendering distance, is basically just rotating squares (second pic). I really love this brush. I do my lineart with this brush and it’s also great to pick up colors and mix and mash them and this is how I do my painting. (I do keep the rendering distance at 5 % though..the second image is only to show the “roots” of the brush)
For the first bold colors I use the simple pen tool. And for the overlay light details I use a light texture brush ..which is kinda like a soft nebula ..so the light doesn’t look too bold and edgy
When you mix up the colors try to avoid black and white but rather go for different colors which are light and dark. In this picture the base color for the shirt is blue. So I used different shades of blue, some purple and pinkish colors and a little bit of yellow and mixed it all up
I hope this helps!