I once asked during an armorer class what changes were involved in the transition from yellow to light blue .40 3rd gen double stack followers. Neither the instructor, nor any LE service/parts people I called later, could tell me why the color had changed or if there had been any other specific revisions.
One factory guy offered an observation that sometimes when the factory changed vendors of a part, or an existing vendor made a minor change in some internal spec (plastic formula used), the color would change denoting the change. He said that not all color changes of things like plastic parts (or springs) necessarily indicated an actual design change or engineering revision of the part.
The light blue .40 double stack followers are the most current. I was told that S&W sometimes sent replacement revised .40 followers to LE agency customers, if only because they requested it when they learned of the change, and it made them feel better. Maybe so. Wouldn't surprise me.
Sometimes it did indicate a needed design change, though. Always best to ask the company.
For example, when the SW9940 followers changed from dark orange to light orange, it was due to a subtle revision of the follower shape, necessitated by the mag body receiving a change in shape adjacent to the mag catch window. The added "flat" in the mag body resulted in the left side of the follower being molded with a 'relief' clearance spot to accommodate the change in mag body shape. The P9940 followers received a similar color and shape revision, for the same reason (going to a lighter blue color for the revised .40 followers, if I remember right).