The yellow "bumblebee" paint scheme seen on the locomotives in the film is not accurate for the era depicted. The yellow paint scheme was first created for the 1949 Chicago Railroad Fair, where D&RGW locomotive #268 was displayed as a symbol of the D&RGW (formerly D&RG) railroad's history. It was used again in the movie, along with several locomotives painted to match. After filming, it returned to normal service, becoming the only steam locomotive to do so in the yellow paint scheme. (The other two that were painted yellow for the film were destroyed in the crash scene.)
In real life, the D&RG's locomotives would have been painted black during this era. Some may have had dark green boilers, but records are conflicting on this, and no color photos exist to settle the debate. Coal-fired locomotives like these were generally not painted in bright colors because, thanks to the soot and smoke from the coal, it would take a lot of extra work to keep them looking clean, so they were painted black or dark grey to make the soot less visible. (There were exceptions to this, of course, such as the Southern Railway's bright green paint scheme.) Older steam locomotives with vivid paint schemes and shiny brass fittings used wood for fuel, which produced a lot less soot.
Locomotive #268 wore the yellow paint scheme from 1949 until its retirement in 1955. It was the last of the railroad's C-16 class 2-8-0 locomotives to be retired, and the only one to wear the yellow paint scheme while in service. It is now on display at the Gunnison Pioneer Museum, once again in its famous "bumblebee" paint (a new paint job - it was painted black for many years before it was cosmetically restored). Unfortunately, there are no immediate plans to restore it to working condition.
The "bumblebee" paint scheme has become an iconic part of the D&RGW's legacy. Although #268 is considered the original "Bumblebee", several other locomotives have worn the paint scheme over the years, and numerous models have been made with the yellow paint scheme (including models of locomotives that never carried it in real life, even for the movie). The Durango & Silverton, a tourist railroad that operates today on former D&RG trackage (including many of this movie's filming locations), uses a similar yellow paint scheme on its passenger cars as an homage to the "Bumblebee" scheme, although the locomotives are painted in their historically accurate black (Period-correct passenger cars would most likely have been painted dark green or maroon).
