Last summer there were some S&W 5906's on Gunbroker with blue paint, and at the time they were identified as US surplus training guns that used a specific type of training ammo. I believe it was also discussed here within the last year.

I believe in the 1990's guns painted a bright color were used in at least one overseas country to make sure people being trained did not confuse a weapon loaded with training ammo and their regular carry gun. If they happened to do that, an instructor could tell immediately. Magazines were also colored.


edit: I vaguely remember that said training ammunition also required different recoil springs, so I would check that in any used gun which looked like that.