The red followers are the earliest ones, followed by the yellow and finally the black followers.
The black followers, being the latest and last revision, are the only ones I use for serious purpose. That said, all the mags and different followers have been trouble free, for me, to date.
I use the orange and yellow follower mags as "range use only" mags.
Yes, the early full size mags from the 645 - the ones with the metal follower and metal base plates will work in the 4506 and 4506-1. But again, if you are using your 4506 as a carry gun or for any serious purpose, I only recommend using mags with the black followers. Hope this helps. Regards 18DAI