Green pigments are actually fairly rare in nature, most birds that look green are actually that color as a result of iridescence, not green pigments. Look at a trogon sometime, you'll know what I mean. And while trogons do live in the most green parts of the world, I laugh openly and cruely at anyone who thinks they're camoflauged.
Camoflauge is only so important to most bears. Outside or north america, sun bears and their lethargic ilk make the perfect real-world counterparts to mr. Pooh. There are no more bears (alas, poor
arctodus ) designed as purely ambush predators, and in North America, nothing large enough to threaten them, so camoflauge would be of only so much use.
Polar bears are an exception, and are quite different in terms of behavior than other bears. They live in a place where the land is basically one color (an obvious oversimplification, but close enough), so that they gain something by being well-nigh invisable. Also, polar bears are unlike most other bears in that they are pure predators.
Now, as for your question the reasons why bears being camoflauged at all (even if it wasn't straight green) are as follows (somebody tell me if they're incomple/incorrect I ain't a world class biologist):
-Bears, especially large North American ones, are really outside the size limit where camoflauge works in reasonable environements.
-NA bears do not pursue a lifestyle where camoflauge would be useful, being omnivores for the most part.
-Most mammals can't see in color anyhow.
It also is worth pointing out that not all black bears here are black. many are brown, and on on island, the're actually straight white, just like a polar bear.
