Isn't smell reckoned to be the dominant canine sense? If that's right, bearing in mind how they say 'hello' to each other .... how far do you want to explore possible motives in doggie decision making? Re: "the discernable shades of green thing" I think I recall something along those lines too, but trying to confirm it just now, couldn't find anything to support it ..... although I can now categorically state, that a bee couldn't tell the difference between Furness Railway or Isle of Wight Railway liveries (unless they've got a better handle on lining out than any published studies, to date, have indicated). Research also shows that humans can distinguish somewhere between 20,000* and 20,000,000** individual shades of colour, according to sources and levels of light .... and whose answer you choose to believe. * All right .... name them ** I'm still betting you couldn't name - not number - 20,000 before running out of eponyms. And you thought 328 'Halls' got tedious!