Is it possible for you to post the settings of the brush preset you use? i really like that style but am never able to replicate it. Just basing off what I see i'd assume your using a softer brush with probably 80-90% opacity with some kind of pen pressure setting since I assume you use a tablet

Also do you use a colour pallete or just randomly pick colours

I'm just wondering since your colours have so much tone and variations in shades that really complement each other, especially in your character hairs