Drem Mk.1 in 1941 had 230V, 250W Angle of Glide Indicators - one only on the left of each runway, showing red / green / amber on approach. It was optimised on 5 degrees.
Later that year, with the advent of Drem Mk.2, a slow rotating shutter was added to make them flicker. They were renamed AAIs and a second was now added on the right of the runway. Later still the left unit was lowered by 0.5 degree, and the RH one was raised by the same amount.
This resulted in green and red when slightly too low, green and green when correct, and amber and green when slightly too high. Clearly two reds or two ambers meant way out!