The green light from a railway semaphore signal showing "clear" at night is really, really dim compared to the red light for "danger". I noticed this yesterday when the Severn Valley Railway was running night trains. I suppose it's logical; the light source is a yellowish paraffin wick lamp and green is obtained by moving a blue spectacle plate in front of it. Filter all the yellow out of a yellow-white light source, and you're not left with much. There's plenty of red light, though.