There is such a big range to choose from these days. It very much depends on the conditions, the environment and how far you want to go really. Think of your location first, maybe take a photo of it and then select shades that blend with areas of cover you see there.
Being comfortable is a big consideration, ease of movement and warmth and keeping dry. You don't want to be fidgeting around if you can help it and you won't sit still for long if you're cold and wet. Go for muted colours that blend with the particular environment. Foliage colours are usually your best bet generally but in the wintertime blue-greys and pale dusky blues can be useful too in bleaker weather, in misty conditions or near water.
Pick something water proof but not rustly and make sure your outline isn't obvious such as standing on a hilltop with your back to the open sky. Check around you and pick some cover that will at least mask your outline from behind if at all possible.
If you want to go the whole hog then
Wildlife Watching Supplies do a range of camo gear that will make you and your equipment pretty much invisable. Costly though. The site is worth a look regardless of whether you are going to buy anything or not, just for ideas, there are more useful tips on there too. Hope that helps a bit
