First, like the title says, around what time did painted roads become the norm? Was it before the highway system? Also, I'm assuming there wasn't an immediate standard to colors and paint spacing. Was this resolved federally? Finally, if there's any cool or relatively unknown facts about the creation of US roadways, I'd be interested.
The Michigan Department of Transportation believes its state hosted the first painted centerline (1911), the first painted centerline on a state highway (1917), and the first centerline markings to designate passing and no-passing zones (1927).
Early efforts to standardize road signs and road markings occurred throughout the 1920s, on both the regional and national level. One thing we don't think about today, with laws mandating lights=>brights and the overabundance of reflective material on signs: early signs could be difficult to read at night, so the shape of the sign mattered. That's why stop signs are the distinctive octagon: you knew by the shape, even without the colors or letters.
The 1935 and 1937 Manual on Uniform Traffic Control Devices made some of the first attempts at regulating centerlines, but they allowed some diversity based on different regional customs (there was more diversity allowed in signage, too). It wasn't until the 1971 MUTCD that yellow was established as the standard centerline color.
The MUTCD website has a very dense historical overview of its development that might interest you!