Ian,
483 made the last D&RGW freight run on 8/31/1968 in regular black paint, and filming of "Good Guys and Bad Guys" began early to mid September, IIRC. So the engine was painted red in the first part of September 1968.
As for when the paint was removed, the Cumbres and Toltec operated the loco in 1970 as delivered from the D&RGW (red paint and all), only major cosmetic difference being the C&TS lettering Mr. Robart applied. By September of 1971, 483 donned classic black without any lettering. Based on the info in this thread ( [
ngdiscussion.net] ) I would imagine 483 was repainted back to black between July and September of 1971. Maybe others who have specific dates can chime in, but hopefully this helps.