I am not sure if it is for UV protection, but it is natural pigmentation found in many paphs.
Many, if not all, coloratum and vini maudiae types develop on the backside of their leaves. Some have this coloring on the entire surface of the backside while most have it toward the base like the one shown in your photo.
By the way, when you describe red coloring is creeping up near the center, I was thinking bacterial rot got started, but if this wine coloring was what you were referring to, you don't need to worry as it is perfectly normal.
Also, I'm pretty sure it was already present, not just showing up all of sudden?
I used to have tons of these as they were so cheap and so common, and each plant is quite different.
So I used to buy a lot and dump out lots of ugly ones. I have kept about 4 that I liked.
They are strange in that the ones I really liked the flowers had too big of leaves and overall plant size, while smaller ones with beautiful leaves had ugly flowers. so I tossed many.
I still don't absolutely like what I have kept, but I kept them for either unique flowers or nice leaves to look at. haven't found ones yet with both nice leaves and flowers. that would be so nice, but I got tired of searching and spending money on them.