I do black with red highlights. Not because "everyone else is doing it", but simply because I've always liked that color combo. Black and green looks cool too, but only certain shades of green. I don't see the problem with it really.
A lot of people use those colors because they look good. With so many limits on how we color our spartans it's understandable to see people with the same colors, especially colors that look good together. That's like complaining about seeing too many people in real life wearing blue jeans and that you're unique so you wear yellow pants instead.
360 games completed 100% retail: Armored Core 4, FF13, FF13-2, Force Unleashed 2, Halo 3, Halo Reach, Mass Effect 2, Mass Effect 3, The Last Remnant, SSX.
Been a white spartan since CE, and then when the included secondary colours went Cyan/sky blue for that. https://spartans.svc.halowaypoint.com/players/Tiny5th/h4/spartans/fullbody?target=large In reach briefly had gold secondary, and currently in 4 using orange secondary as don't like the gold.
I've been orange since Halo 3. I mainly do it because so few games have color besides gray and brown, so I like the idea that I can look bright in Halo.
It was particularly funny in Halo: Reach on the stealth mission with Jun. He was all colored and geared for sneaking alongside that ledge in his dark green and ghillie-suit torso, and then behind him was a seven foot tall bright orange guy.