S&W does color-code many of its followers, but it's not like there's an exclusive color for every caliber. The 3rd gen .40's are presently blue, but the M&P 40 and SW99/990L .40 followers are 2 other different colors ... and there's been some variation within each of those other model line followers, as well.
The revisions within each caliber
may be reflected by a color change, but then maybe not, too. (The present M&P 45 followers that are listed as revised parts are the same color as the original followers, and I'm unable to see what difference, if any, exists with the naked eye.)
Also, as I've been told by folks at the company (as an armorer), sometimes they'll list a revision, or maybe even change the color of a part (like a follower), simply because they change vendors for that part. There may not be an actual engineering or design change, but just a vendor change. Maybe even a materials change by a vendor. The list has varied as it's been described to me over the years.
One time when I was discussing such things with one of the guys who used to be in production, as well as having served as an armorer instructor, the subject of 3rd gen .40 followers came up. I asked about the difference between the yellow and blue followers (as I've done from time to time, just to see what I can hear). He wasn't able to give me a definitive reason for the color change of that model's followers, to the best of his knowledge (about 30 years) and didn't know what, if any, difference was involved other than the color change.
Then again, I remember when I came across some of the orange 9mm followers used in the 59XX series mags that had a distinctly different shape and shade of orange color than later 15rd 9mm 59XX followers ...
... and I never was able to find out the reason for that uniquely shaped convex top contour of the follower, or even how long it had been in use (although it doesn't seem to have been around for more than a couple or so years, since we started seeing the "current" orange 15rd 3rd gen 9 mm follower shortly thereafter. It sure worked well in my guns, though.
On the other hand, I was also told some years ago that when the company changed the 11-rd 4006 mag follower from yellow to blue, some major LE agencies felt it was prudent to change their existing mag followers over to the new ones. (The 4013TSW .40 follower is the same blue one, but if you ever come across an older single stack 4013 - built on the 45 frame dimension - take a look at its .40 follower color, or the color of the CS40, for that matter. Things can vary.)
It gets even more confusing when you start to try and track recoil spring colors, or the lack thereof.