Guys,
If you're in a place under circumstances that make you think a "tacticool" or military style backpack is going to mark you, and by mark I'm guessing you mean draw unwanted attention and implying being robbed or assaulted, then you'll have the same issue if you're carrying a pink Kelty pack.
If things have gotten so squirrelly that someone is going to target you to steal your stuff they're going to steal ANY packs.
If you're worried about official types, I sincerely doubt that they much about the color or style of pack you're carrying, they're going to <probably> be more concerned with crowd control and unless you're waving a firearm around or otherwise acting like a threat you're going to be just another face in the crowd.
What I do recommend is that you not garb yourself head to toe to look like military personnel. You know the look. If you've been to at least one gun show you've seen the someone dressed up in a ersatz soldier suit, either surplus US or Foreign, trying to convince you his myopic gaze is actually a thousand meter stare.

Don't be that guy plus an alice pack, looking like your out playing jungle patrol with your airsoft friends. You'd probably just be the butt of jokes for all that see you if you do, bit there's a slim chance someone might think you'll do something stupid with all that kit and take preemptive measures.
Just get the most durable, comfortable, functional pack for your needs that you can afford and make sure you have it fitted to you properly and use it periodically to keep your stabilizing muscles in shape.
I'll repeat this again because it's more important than many people think- Make sure you get the pack properly fitted to you. If it isn't you'll wear yourself out faster and be more prone to injury.
The HPG packs and pouches my wife and I have are mostly in shades of coyote brown, with some OD stuff. Civilian boots, a brown smock from Arktis, clothing is all civilian looking stuff. It's not going to be a problem if you look like you went shopping in the bargain bin at an old school Army Navy Surplus store, just try to avoid looking like Walter Mitty out for a stroll.
TL:DR-
Don't worry about .mil style or colored packs. There is no "gray man" advantage to avoiding them in favor of "civilian" packs.