Each railroad sets its own signal standards. That is why there is so much diversity in how wayside signals look, as to height, size of backdrops, hoods, location of number plates, other plates, such as G's, or illuminated letters, such as S for "take siding". The actual signal indications and aspects displayed are also strictly the choice of the railroad, and there is some variation there.
However, on a 3-lens signal unit ("signal head") the colors are usually green-yellow-red, from top to bottom. When they try to save cost by avoiding the use of two signal units to give aspects such as yellow over green or double yellow, then the single 5-light unit can be arranged in unusual ways so that the colors are separated enough that the Engineer can clearly see the indication. Most railroads do not do that, and use two separate units to put distance between the two colors displayed, enabling them to be clearly seen as separate colors from an approaching train.
Chesapeake and Ohio, I am told, normally arranged its color light units red-yellow-green, top to bottom. That was the standard they established.
