The meanings of several different railroad lights are green for "go," red for "stop" and yellow for "proceed with caution." Two colors can also be used together to create a signal, such as yellow over green for "advance approach." There are two main types of railroad signals, permissive and absolute; permissive signals provide protection while absolute also provides control.
As of 2015, railroad signals used in North America use a set of colors that were standardized in 1905. Yellow is used to warn engineers of speed reductions and is used when the train is moving at a slow speed. Red is not only used to indicate "stop." It can also be used as a placeholder light to let engineers know that the railroad signal is working.
A lunar white railroad signal is a blue filtered light that removes all yellow. This is used to indicate that there is a restricted-proceed condition ahead. A lemon-yellow light is used when a high-visibility color signal is needed, such as during foggy conditions. A blue signal is used to let crews know that equipment on the track behind the signal should not be moved, and it is used to indicate an intervening track between the signal and the track to which it applies. Purple signals are obsolete as of 2015.