Snow over drizzlin grey. Because the colours are move vivid with grey, green AND white?

I am not an expert but I think the "turnng" of the leaves depends mostly on the length of daylight whereas the "falling" of the leaves depends mostly on temperature and moisture. Any experts out there? Ranger1 or one of the other naturalists/science types? or 'Cats who can find anything on the net in 0.0001 seconds?