good for you. that's an important step. what part is turning purple? the head is meant to be purplish, even the bottom of it.
you can leave the foreskin back all you want. it isn't comfortable for most of us, but you can do it. the rubbing drives me out of my mind, until I cover the head again. why would you want to?
If the head is purple because the head is purple, that is a good thing. That is how it is meant to be. If the head is a darker purple, because the foreskin is so tight it is cutting off the circulation, that is a bad thing. Why would you want to leave the foreskin pulled back? You can do it, but it is sort of like holding your eyelid open.