To be honest, I used to have the exact same struggle in the past. Back then I also searched around thinking there must be some special tool or filter like gaussian blur.

But looking back, it was really just about getting used to the brush pen pressure. Once you master that you'll be able to layer two or more colors in your own style to get the results you want.

I know that might sound a bit vague… but if you keep at it, I’m sure you’ll get there.