hi erica, I think most would agree, ask anyone on here and they will say it's normal, mainly down to genetics and I agree with that, some seem more likely to do it than others.
We could leave it at that and I don't wish to confuse you. Some would say so why try? Why not, if we discover something new it might be worth getting confused once in a while.
I actually did a bit of experimenting when it happened to my phal and I managed to get rid of the purpling in the end. It took a while though.
Anyway I took a quick snapshot, in hindsight it's not the best, it's growing better than it looks on the picture, I took it at a weird angle to show the underside of the older leaves and I tried to get a bit of the new leaves which are now a nice lush green again.
Just in case you were interested. I cannot tell you how I managed to do it unfortunately. I have copyrighted that information. But maybe there is a member here who also knows and is able to tell you.
I know many do insist it is completely normal and I'd agree but then I have managed to make mine grow out of it. So I will just add the picture as evidence of what I am talking about. Pictures speak a thousand words right?
Sorry I can't be more help, I wish I could.