* Early 1900s: Cars were typically painted a single, dark color (black being most common) primarily because it was the easiest and most durable color to achieve with the paint technology of the time. Choosing a different color was not really an option.
* Late 1900s - early 1910s: As paint technology improved, other dark colors started to appear, but these were still limited options, perhaps offered as a slightly more expensive "upgrade." It wasn't a wide selection.
* 1920s and beyond: The development of more advanced paint technologies and mass production techniques allowed for a wider variety of colors to be offered, with the choice of color becoming increasingly common in the 1920s and 1930s. This wasn't a sudden shift, but a gradual increase in the number of available options.
Therefore, there's no specific year. The ability to choose a car color emerged gradually throughout the 1910s and 1920s, becoming a more standard feature by the 1930s.