It was 166 pages long and recommended a yellow stop sign with black letters." - New York Times, December 2011. One of the things they had to get away from was that the stop signs looked like yellow warning signs. Also by changing it to red in 1954 made it consistent with the red signal lights.
Here is the kicker. Why octagon? Why not square? They needed a shape that would be recognizable as a stop sign so it couldn't resemble another sign. Several countries have actually adopted the stop sign with the red and white. Japan uses a triangle(because they want to be different) but still use red and white. Now when you see an octagon sign you should be thinking Stop!