This is a special time. Sometime next month, NASA's Human Space Flight Review Committee will announce its conclusions on whether NASA should pursue its spaceflight goals as they are currently planned. It may be that by September, ideas of a crewed Mars mission in 2030 will have been relegated to the same dustbin as the Apollo-tech Manned Venus Flyby1 or the Delta Clipper. Until we know the shape of the future, though, there's plenty of speculation to be made on what challenges that mission may face.
Humans can't walk in shirtsleeves on the Red Planet, but neither would they wear off-the-shelf spacesuits either. Modern spacesuits, as found on the shuttles and the International Space Station, are meant for a very specific purpose - to maintain a comfortable pressure and insulate its wearer from temperature extremes, radiation dangers, and other environmental hazards present in the vacuum. They'd be perfectly functional on Luna, but Mars is not quite as harsh as that pitted, airless world. Most likely, a Mars expedition would take with it specialized Mars suits optimized for conditions on the Martian surface.
There are a few efforts currently underway to design a functional and effective Mars suit. In August 2004, the Sydney Morning Herald reported on one of them, the MarsSkin project, attempting to simulate Martian suit conditions on Earth. At the time its prototype suits were being tested in South Australia, and were "elasticised to counter the effects of the lower atmospheric pressure on Mars." This is most definitely the sort of research you want to do, and do right, before you leave home or you will look worse than Doug Quaid after he got sucked out the alien chamber in the Pyramid Mine.
There's one detail that could easily be overlooked in the shuffle of assembling an effective personal environment, though; the color. The MarsSkin suits photographed in the article are deep blue, close enough to black in shadow. Visibility is important everywhere, but the hostile environments we find throughout the solar system make it key. If a Martian traveller takes a fall and finds the suit's emergency beacon broken, say, a noticeable paint scheme could well be the difference between death and survival.
Popular depictions of Mars don't have much agreement. The first major Mars-focused movies I thought of, 1978's Capricorn One and 2000's Mission to Mars, use ordinary white spacesuits that would not look one bit out of place on the International Space Station. Red Planet, also from 2000, has its astronauts wear black suits - which could be an advantage in terms of heat retention - and 1990's Total Recall begins with a dream sequence where characters wear khaki suits with disturbingly fragile faceplates.2
For a while I thought MarsSkin was really on to something with their blue suits. After all, there's not much that's blue on Mars - all but, it seems, the sky. Although it appears salmon or butterscotch through the cameras of our probes there, the evidence I've found suggests that its true color is the same familiar blue as on Earth. This question won't be settled once and for all until people actually see it with their own eyes.
I brought the idea of this post up with a friend, and she didn't waste any time in coming up with a preferred color: green. And, you know what? I think she's right.
Its sky aside, Mars is red, thanks to the plentiful iron oxide dust. Not only are red and green close complementary colors, there is absolutely nothing on Mars that is green that would not have been brought there by humans. There are no Martian trees, no Martian grasslands, no Martian jungles. The smallest swatch of green in that rusted desert would be all the more starkly visible for its loneliness.
Should NASA's human spaceflight ambitions go ahead, and should humans walk on Mars in my lifetime or anyone else's, I think it would be best if the color they wore was green. Our presence there alone would make it appropriate - bringing living nature to a lifeless (according to current evidence) world.
1 Although, James Nicoll had a rather sensible objection to this plan.
2 2009's Watchmen also had scenes on Mars, but Dr. Manhattan is above such petty requirements as "air."