Sketch12211201

Original art by kancutboy.deviantart.com/
Seriously I don't think the original needs improvement. Your technique adds character to the portrait.
But maybe you are looking for something different. 
I took away some colors, the harsh lines using the eraser and the smudge tool. You can see the slight differences here and there.
And did some curve adjustment in photoshop.
When you apply colors to skin, the smallest opacity is the key.
Hope this helps.