One must remember that the colors for signals changed around 1920 or so. Until then, Yellow was not accepted. White was clear, Green caution, and red "danger," of course. Switch stands are considered fixed signals in most rule books.
For this reason, I cast doubts on the harp stand targets in the old days being yellow. More likely they were white.
It has always been my understanding that the colors on harp stands were not used to indicate switch position, though the lever's position would tell you that. The colors were mainly for visibility, which makes sense with red/white and so on.
If you look outside Colorado, you'll see some that some targets looked like a crown painted red with a white circle in the center; some with two diamond-shaped targets one above the othere and one white, one red.
Skip Luke
Switchman (ret): Grand Canyon Railway;
Engineer (ret): Georgetown Loop RR Inc., Sumpter Valley Ry., Monticello & Sangamon Valley;
Dispatcher(ret): Illinois Central, White Pass & Yukon, Burlington Northern Ry.
Mariner, Musician, Miner