The blue followers are the "current" (latest) color.

I've asked a few guys (reps, armorer instructors, customer service LE contact, etc) about the color change from yellow to blue for the .40 followers, but I've never really gotten a definitive answer.

I've not been able to eyeball a difference, like you can sometimes do with other follower revisions, but that may only mean that whatever revision or change was involved, it's not something you can see with the naked eye.

I've also been told that sometimes S&W may change a vendor, and if they do, that might involve a color change (plastic or paint) to ID parts from the different vendor.

Or, it's been speculated it might involve a specified change in the material used (plastic formulation), for some reason known only to the engineers.

Whatever the reason, I've not been told that any yellow follow .40 followers "must" be replaced with blue ones. If you ordered new ones, though, you'd get blue.

I was told that one large agency changed out all of their yellow .40 followers for blue, simply because it was the newest part and that's what they wanted to do.

__________________
Ret LE Firearms inst & armorer