Hi jacek!
I am from Japan.
I use a pen and tablet small size.
I draw all of these pictures `Black Inc` soft.
You can draw a lot of colors by using layer and layer option `normal``screen``multiply`and so on.
My trick is painting 2 layers with different colors.
For Exp.one laye is blue `normal` another layer is yellow `overlay`.
And draw many color variation on the blue layer.
You can draw many attractive color variations.
The trick is using layer option `normal``different``Hard Light` and so on and color option`s`normai``Additive``Multiply``Subtract`
Try many colors and using your imagination!
And you should read many art books.