I came across several magazine followers for a Model 4506 in my parts box. Some are yellow, some are black, and some are orange.
Is there a reason for the different colors ?

As the magazines changed slightly over the engineering changes, the followers changed. First ones were stainless followers, then orange, the yellow (some even said “this magazine must have a yellow follower”) and the latest is black.
The black ones are backwards compatible with all the previous versions, and work best in my experience in all versions of 4500 magazine.
Its the same follower in the 945 also.

I have magazines with both the orange and the black followers, no problems, so far, with any of them. That being said, the black follower was the final design and is the desired follower for magazines intended for serious uses.

If there is one 645/4506 magazine that I would not use for serious use, it would be the very first design that retains the floor plate with a metal spring tab on the right side of the floor plate. These have been known to fail at the most inopportune time.

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VCDL, GOA, NRA

I have a few of those mags and use the modern floorplate and retainer, as well as the black follower with those. Solves any problems for about $11.