There's no way to know by cart color. Some people even switch out cases for other colors, although it's not common.
It's actually usually really easy to tell if something is a boot. Look in the little slits in the side of the cart and look for shoddy soldering. Most bootlegers don't do factory soldering jobs so you can almost postively tell right away if it's not a factory job. Also, just look in there for any suspicious chips with windows (eproms). That doesn't always mean it's a boot or not, but if they're all eproms then it's definitely a boot.
Of course, you can't look in the slots at all in the pic, so then asking in a forum like this is a good choice.
My vote...
not a boot, considering the other carts he's selling are not boots.
Don't be so paranoid though. Like it's been said, boots aren't that common. There are some games where boots of them are everywhere (Puzzle Bobble, Super DodgeBall, Garou, and the newer games), but for the most part, you won't be running into boots that often.
<small>[ July 29, 2003, 10:24 PM: Message edited by: biocat ]</small>