Edit: Marked as solved. See Post 4 for clarification.

Test System: Late 2013 Macbook Pro, MacOS 10.12.6 (Sierra)

I've been playing WC3 on my Macbook for a few days and it plays well, but the colors are just off—notably, the
sky background is way too bright.

I've tried using the "Game Setup" option from GOG's launcher, but even the lowest gamma setting (Ctrl+LeftArrow) leaves the skyfield an ugly, bright shade of blueish purple instead of the near-black it should be.

The wrapper uses Boxer behind the scenes, so I was thinking maybe there's some Boxer config I could change to adjust in-game gamma (?)

I'd consider trying a manual DOSBox install, but it shouldn't be necessary if the game has full Mac support.

Side Note: Sketchy "Game Setup"
As an aside, the "Game Setup" is a bit broken, in that it requires manually resetting the sound card options and savegame directories to get the game up and running again ("Soundblaster 16" (IRQ=7) and "c:\", respectively, will get you up and running).

Visual
Just as a follow-up, I wanted to mention that I moved the game over into a full Boxer wrapper to see what settings could be tweaked, but it looks like Boxer removed all the SDL options (notably the "output" option) so there's not a lot that can be done to adjust the visual appearance.

I'm going to try changing out the emulated machine, as some have reported certain machine options tend to render darker. I'll report back if that changes anything.

Audio
Also, to follow up on the "Game Setup" audio—while "Soundblaster Pro" works well enough, the sounds are a little tinny. "Soundblaster 16" seems to be the better option, as that's what Boxer is emulating behind the scenes (updated my above post to match). I had to set an IRQ of 7 for the sound effects portion instead of the default 5, but otherwise it was pretty straight forward.

Update: Adjusting the emulated machine didn't really accomplish anything but degrading performance.

It doesn't look like there are any adjustments to be made within Boxer to correct this issue. I did find a simple enough solution, though.

Since it's a pain to mess with Apple's color profiles directly, you can approximate the correct darkness by using MacOS's Accessibility options. Assuming you're using the standard color profile—using System Preferences, in the Accessibility section, under the Display options, increase the Display Contrast to nearly the first full tick mark (about 15%). That should leave you with a comfortably playable game but a much more reasonably dark space environment. Depending on your screen's brightness, you may need to adjust it down a bit for better visual comfort in station environments or long dark cutscenes.

I'm just going to go ahead call this solved, as I don't see any way for GOG to fix this while continuing to use Boxer.

Though if anyone comes across something better, by all means, please share ;)