CA does design the maps, but the engine, lighting, and miscellaneous other assets obviously come from 343I. It's why CA's other efforts (Halo 2's Blastacular Pack, Reach's Defiant and Anniversary Packs, and all the Halo 4 DLC), look congruent with the game they're making in terms of lighting, color, and general aesthetics. The H2A maps look straight up like they use the (perhaps modified) Halo 4 engine.

: Also, graphics and art has changed in games as games have
: increased in fidelity.

The excuse that devs needed to switch to a "Real is Brown" palette because of increased graphical fidelity is an excuse that's been around for a while. It's still a BS excuse. Gears of War did the whole "Real is Brown" thing, yet Halo 3 comes along the next year with one of the most vibrant entries in the FPS genre. COD did the whole "Real is Brown" thing as well for years despite being a graphically primitive game for a seventh-gen series (the series has used what is in effect a heavily modified version of the now 15-year-old id Tech 3 first used in Quake III). Incidentally, Gears 3 actually managed to add at least a little more color to break up all the browns and greys, and Black Ops II had a surprisingly colorful multiplayer component (never played the single player; played the MP at a friend's house for a bit). Even in this generation there's games with amazing visual fidelity that also are bursting with color. Infamous: Second Son is very vibrant, Killzone: Shadow Fall has a good variety of colors (esp. in the Vektan levels), and of course nearly everything Nintendo does is amazingly colorful. Even Destiny has splashes of color all over the place, and even though some of the environments look rather monochrome (and the Moon and Mars aren't the most colorful places in real life), they still don't look muddy or have harsh lighting. Whether it's Infamous or Killzone or Mario Kart 8 or Destiny, there's plenty of games that don't make my eyes bleed and/or look like they've been covered in a layer of grime.

No, going with a muted palette consisting exclusively of earth tones and greys and harsh lighting is entirely an aesthetic decision, not anything forced on them by changing tech. "Real is Brown" has been a thing for a good long while now, and all the games that use that trope have contemporaries that use just as advanced tech but look far more colorful.

: Maybe this is why Bungie did this with Halo Reach which kicked off
: the whole change in color palettes for Halo.

Reach is still vastly more colorful than this. I still play Reach at least once a month, and most of its environments look wonderful in terms of lighting and color. Every time I go back to play it, I'm reminded at how much better it looks than Halo 4 in almost every regard.

: Luckily, you can play all the old maps in their original colors if you don't
: like the new versions and their colors.

I actually do care more about how they play, and the core gameplay for the H2A maps looks like it'll be just like how Halo 2 should have played ten years ago, e.g., having the AR as part of the weapon sandbox, giving the sword a battery, eliminating glitches, having a more consistent shotgun (presumably donut spread is gone and the pellet count is consistent), and I would assume melees are more consistent and the plasma pistol charged shot's tracking is reduced (jury's still out on those two). I just wish the maps weren't such fucking eyesores in terms of color and lighting. Hopefully it isn't as bad as screenshots and internet videos suggest and it actually looks better in motion on my plasma screen.