Perhaps its not to do with hand speed or anything. If you are making things dirty looking at the end but it heals good? Whats the worry? Them photo tattooists there work is shit when it heals great for the photo but thats about it. Perhaps you just need some colour theory the video russ Abbot does is great for that because you might be making things dirty because you are using the wrong colours together. ... when I first used to airbrush my work look dirty cartoon characters looked they had been down a coal mine so I have a rethink then started to used candy colour for the darker tones then the work improved over nite...