The 1948 MUTCD settled the early center-line color debate in favor of white.
The third edition of the Manual on Uniform Traffic Control Devices, issued in 1948, established white as the standard color for center lines. It also recommended yellow for double center lines on certain multi-lane highways and for center lines marking no-passing zones. Those exceptions help explain why later American road markings can seem inconsistent with the headline answer.
The MUTCD grew out of efforts to standardize signs, signals, and pavement markings across the United States. Its first edition appeared in 1935, after earlier urban and rural traffic manuals had used differing conventions. Uniform markings were considered important because drivers needed to recognize a road message quickly, without having to interpret a state-specific system.
The color scheme later changed. The 1961 MUTCD made yellow mandatory for the two exceptions that had previously been recommendations, and the 1971 edition required yellow center lines to separate opposing traffic while white lines separated lanes moving in the same direction. Thus, white is the correct historical answer for 1948, but not the general rule on modern U.S. two-way roads.