I've heard that sativa's tend to be lighter tints of green and indicas darker shades. That's generally been the case in my experience, although off the top of my head I can recall one strain, 'afghan black' that is quite dark but makes for a pretty heady high. Come to think of it, I'm pretty sure I've smoked light-green grass that made for a heavy-body high too, so...

So yeah, I'm sure there are at least a few exceptions to that 'rule.'