I recently posted a question, as to whether certain HV crossarms in old photos, had yellow paint. At left, is an image of one of the lines in question (old transmission line in San Francisco). The crossarms appear smooth, as if they may have a coat of paint. Yet, the tone is dull, not much different than the unpainted wood of the pole.
The problem is the old nitrate film, that was used. Nitrate was quite sensitive to blue light, but had very poor response to red or yellow. So, on the right, I did a test, using an enlargement of a CGI image, that I posted recently. The top right image is the original, color version. And, the bottom right shows a simulation, of how that same scene would have responded to black-and-white nitrate film. Notice how the brilliant yellow of the sign, is now hardly distinguishable from the dull, weathered wood. So, the crossarms at left, could indeed, be yellow. By that same standard, the signal arm below, may have been painted red, due to its dark, smooth appearance. Red often appeared almost black, on nitrate film.
I would also like to thank Gary Yarborough and Paul Greaves, for their research on this question. They provided documents, proving that HV arms in California, had to be marked, at the time that these early lines were constructed.