I have no idea how often they swap, but generally speaking, Federal agents unholster their weapons in public very rarely. So, there is probably not as much need for frequent rotation compared to the military.
They may have a "colour of the day" and routinely swap them. Hard to know how they're affixed, though - they don't look very swappable.
Could be something as simple as hook and loop tape. One side is permanently/semi-permanently affixed to the slide the other side gets swapped based on x criteria. That's how I'd do it. The military loves velcro.
True, but the photos don't seem to show the depth that hook-and-loop would add. That, and even the milimetre that hook-and-loop would add could impede draw and reholstering.
I was in a non-US military and we used electrical tape - two or three strips around rifle magazines.
It would be crazy if they didn't have a color of the day, but then again, humans are not always rational, and it looks like it's not the type of tape that comes off easily--though it's impossible to tell just from a photo.
Yeah, that's my take too. If it's removable/swappable, it'd have to be something that never impedes drawing or reholstering, which makes me suspect it's more permanent than hook-and-loop, at least.
I've had strangely a lot of experience with stickers and sticky tape, and the material the sticker is made of can aid removal even if the glue is sturdy: paper stickers tear, but thicker plastic stickers will hold together as you force them away from the glue. Alternatively, the glue could be water soluble, or perhaps soften with heat.