I went for a look around the Palm House in the park this afternoon, saw some beautifully exotic blooms inside and some lovely bright flowers like this one in the gardens outside.
Seeing red and green got me thinking about the saying because they're so often together in nature. Seems the most likely explanation is that it's about ships that pass in the night. If you can see your red (or green) light and the other ships red (or green) light, you are passing safely in opposite directions. If one is red and one green, you are travelling in the same direction, possibly at an angle to each other, and may collide. It makes perfect sense now!
