I don`t know a great deal about vintage American holiday lighting, but suspect the dissapearence of the yellow bulbs was due to lack of popularity. Yellow-coloured low wattage bulbs tend to look very similar to opaque white low-wattage bulbs, because the inefficient tungsten filaments burn at a more yellow-orange colour than larger lamps. Perhaps consumers preferred multicolour sets with orange bulbs because that colour was much further away from the whites? It may also be because yellow is so much brighter than red, blue and green. Together with white bulbs, the other colours could be somewhat drowned out?
I have noticed that yellow lamps seem to be preferred in Canadian sets in place of the oranges, but could not tell you why. It could simply be a cultural difference. Perhaps Canadians in general don`t like white lamps in their multicolour sets, or if they did, just weren`t too bothered about the similarity between white and yellow?
British medium-bulb sets generally had white, yellow *and* orange, but went out of popularity in the 70s. We don`t have an equivalent of the C7 or C9 lights, our mains-voltage paralell wired lights use full-size 25 watt bulbs and are for outside only! So it`s just miniatures really, which usually have orange bulbs mixed in with the red, green, blue and pink, but no yellow and not usually whites either (unless it`s a Pifco set).
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