Well - I have one such a filter too, but keep it on the viewing lens (I actually do not know why
).
Anyhow - the filter that is supposed to cut the UV should be eiter clear (== colorless) as you do not see the UV or if it should act as a skyfilter than it might be (and actually is) slightly red when viewed agains a white backgorund. This is as expected - if you cut a small part of the blue color of the light spectrum (let's say a white one) - the resiluting spetrum will be slightly reddish. The more blue you cut the more reddish the remaining light (filter) will appear. Look at any of the 81 filters (81A, 81B etc) - they are reddish as they are used to warm the photo up if you photograph in a colder light (open shade or at higher altitudes).
I think you should be more concerned with the cotaing of the filter. The Rolleiflex filters are mostly rather old and (I guess) single coated. I have even one filter called "Agfacolor" from Rolleiflex that is not coated at all. Lenshood should be your friend
You can get new filters from Heliopan, though they cost a bit.