Battle Dress Uniforms are the combat clothes of the fighting forces of the UNSC. They typically feature camouflage patterns and are made of rugged materials, they also have patches affixed to them to show name and rank insignia.
BDUs are also availible in the real world although the specific patterns used in-game do not seem to have a perfect real-world counterpart. One of the difficulties in matching these patterns is that since they only appear on television, or computer, there is no way to view the camoflauge in the real world, and their colors are always going to be affected by what devices you're using to view it, and what the settings are.
One argument is that the "coffee table" books such as "The Art of Halo", or the various Game guides show a printed image that is presumed to have the most accurate colorations, as it originates from Bungie graphics. This presumption still has it's flaws, but at least it's a common starting point for everyone. Be advised though that many people will prefer to go with the colorations that their particular game/computer displays, regardless of any other "standards." The Halo Costume Canon page further discusses these concepts.
However, one common feature that seems to be part of the Halo 3 environment is that the BDUs are of a high contrast type of camoflauge. Presumably this was to make it stand out as camoflauge rather than abstract shadows on the characters.
It is recommended that if you intend to wear a Marines costume within a group, you should get together and discuss what pattern is to going to be used, and then try to get everyone to wear the same type.