Season 5, Episode 1: The Gun Original Air Date: 13 September 1966 To knock out a German bunker, the squad must drag a cannon over a mile of rough, enemy-infested terrain.
I was 9 when the series started and watched it in real time. I don't recall seeing any episodes in color, and it is a qualitatively different experience in color. The characters' faces are more interesting, the action holds attention better and the backgrounds are more distinctive.
Combat purists might favor the black-and-white which engenders a grittier noir quality due to so many episodes showing stories set during the night.
I don't know if the whole last season was shot in color, but the Amercian Life Channel showed this episode on 20 July 2007, and they are running episodes in rough order.