Red and white were designed to simulate a small semaphore signal (or at least that is what they told me when I was training) and the newer red and red ones are just an improvement on that design. The brand new LED ones are a significant improvement, LED is fantastic technology in my opinion, superior in every way to bulbs.
I presume some of the reasoning would have been cost too. Back when they were first replacing semaphore discs or arms was the cost of a colour light signal more in real terms than it is today? Most of the old designs were indeed three points of a triangle, two white and one red. There are signals with four lights on them though, two whites in the "off" position that a semaphore arm would make, and two red ones in the "on" position. Does anybody know whether these were a later design or just brought in on a region where they had more money?
