if you get hair dye on skin (face,neck etc) a gentle nail varnish remover takes it right off- i use it all the time, then once you've finished dying your hair, and you've got time, wash the skin where you wiped the nail varnish (use cotton wool balls to apply nail varnish- and rub gently).as for the scalp- step one: when your chosen dye is ready to wash off, rinse till water runs clear- they say that for a good reason- also if its one that says add a little water and lather- then do that really well, it stops spots of dye on scalp staining.
lastly, a really good intense shampoo- as in, use finger tips and massage scalp well.