I have both the PM and the GM. I find the GM does fit noticeably more (to me, it's really the difference between carrying a water bottle or umbrella or not, since either size fits my other stuff (wallet, small cosmetics pouch, keys, sunglasses, Kleenex). The GM and PM weigh about the same (although the GM fits more so will be heavier loaded up) but feel different to wear, largely because the GM is significantly taller than the PM. I'm 5'6 and had the strap of my GM shortened 8 inches because the bag is so much taller than the PM so it felt like it was hanging down too low. Somewhere on this thread there are photos that I posted showing the difference. If you don't like digging in deep bags for your stuff, the PM may be better for you.
As far as colors, it really depends on your personal taste and warddrobe, but I'd suggest Colvert, Blue Sapphire, Blue de Galice, Rouge Grenat, or any shade of orange (orange, Orange Poppy or Feu) as a non-neutral that can work well in all seasons. H does colors so well, just pick one you love that goes with your wardrobe!