red and green buses at night

I remember when I was growing up in Bushey often catching the bus at night. There were several types of bus which used the same stop, among them the 306 which was green and the 142 and 258 which were red, in all other respects the buses were identical. Before an approaching bus was close enough to read the number it was difficult to determine under the street lights what colour the bus was but as soon as it was close enough the colour, green or red as appropriate, would be obvious. One day, arriving late to find a bus already at the stop, I was queuing up when I realised, not having noticed the number and unable to see it from where I was, that I didn't actually know which bus it was I was about to board. Try as I might I just couldn't determine what colour the bus was (the green 306 was no good to me) even though it was right in front of me. It wasn't until I asked someone what it was that the bus suddenly took on its "rightful" colour, having "become" red it then became impossible to recapture the original ambiguous hue.