Basic black is always good. Just don't hang a lot of stuff off it.

For years I've used an series of ALICE packs, OD. It's on a civilian frame, which helps, and the old one had a bunch of stuff like Greatful Dead bears on it in the form of patches. That went through customs on the Canadian border many times, and it never raised an eyebrow (Dead Head hippie looking pack, long hair, goatee, it's all camoflauge. :P ) Unfortuantely, I had a seam failure on the pack (it WAS produced the same year I was born, go figure) while test loading it last spring, so the new ALICE bag never got the bears. I also modified a spare tire cover for a Jeep to use as a pack cover.

I would say that if you only have a slightly military looking pack, you are probably fine. I do think that if you are going to wear utilities, no camo, no black, no OD, plain tan or grey or navy blue. The risk is looking TOO military. I think there are enough miltiary surplus backpacks out there that they won't be seen, even in Time Square. That being said, the new BOB just got here today, and he's woodland THE Pack that is going to have pouches hung on him. But I'm also not urban. :P