I would have to agree with Mr. West. Historically on the DRGW, tall stands with red targets indicated a switch on the "main track" and was always lined for the main unless specified differently in special; instructions. Switches on auxiliary tracks or what we now call "other than main track" were historically painted yellow and usually lined for the through route. Green targets indicating lined for main track, in addition to the red, came along later.
With that said, reviewing my DRGW Timetable for select years from 1949 - 1968, a RGS 1947 Timetable and various DRGW Rule books, I find no reference to color of targets and use.
Unlike today, railroads at that time identified "yard limits" (main track) as a general location (Durango) and not always specific between two points. As Mr. West explained, the track leading to the left that crossed the river to the RGS connection and to serve the smelter an other industries may have been considered a part of the yard limits, and therefore a main track. The red target would indicate same for switch position and remained in place long after. Weather this track was considered yard limit, then, or in later years remains speculation on my part until I can find a specific special instruction indicating one way or the other.
My 2 cents
MD Ramsey