Those of you who are interested in the historical accuracy of highway markings and signage should know about the "Manual on Uniform Traffic Control Devices (MUTCD)", which is a series of fascinating documents by which one can trace the evolution and timing of highway safety design (and are also a good source of artwork for homemade signage). The documents date back to 1935 and are freely available for download at the following site:
https://ceprofs.civil.tamu.edu...ns/MUTCD-History.htm
Also to be found there are a series of scholarly articles published by the Institute of Transportation Engineers, tracing the often confusing evolution of highway standards. A perusal of these papers will reveal that the real story is often far more complex than simple statements like "yellow lines were introduced in about the year xxx" could ever capture.
For example, the following describes the state of the art as of the 1948 edition of MUTCD:
Markings
The standards for center lines and no-passing-zone markings sparked so much discussion that two special polls of state highway departments were required to resolve the controversy. Part of the reason for the controversy was that the new standards negated those that had been adopted by the American Association of State Highway Officials (AASHO) in 1940. The JC eventually decided that white markings were to be used for all applications except for double center lines on multilane highways and the barrier line of no-passing zones, for which yellow was recommended. White continued to be permitted as an alternative to yellow. Solid lines were specified where crossing was prohibited and broken lines where crossing was permitted. Although the 1935 manual had identified the benefit of using pavement edge lines, the 1948 manual recommended against the practice, asserting that experience had shown they were easily mistaken for center lines or lane lines.
So, in other words, as of 1948, (a) yellow center lines were recommended but not mandated; and (b) white edge lines had been adopted and then abandoned. Interestingly, the 1961 manual reversed course again on white edge lines, and mandated a solid yellow line to the right of a white center line as the standard for no-passing zones.
So, there was pretty clearly so much confusion, changing of minds, and overlap of old and new that you shouldn't feel too guilty doing almost anything that you think looks good on your layout.