How can I fix a macOS screenshot of an X11 window that’s bleached / white / too light?

This looks to me like a display profile mismatch - specifically the gamut curve would do this.

Open your macOS system preferences and search for "profile"

I would change these around a bit and see if you can get the images you want from the Screenshot app. Specifically, choosing a different display profile or running the Display Calibrator Assistant to choose your specific values should help greatly.

If you’re sure macOS isn’t causing this, you’ll want to look at your Xorg or server preferences for color space if you need macOS to do the screen shots.

• https://tronche.com/gui/x/xlib/color/

Alternatively you might try XScreenCapture, xwd or similar to capture the bits from X11 directly to an image file.

I use xwd to capture a screenshot in xwd format, then display the screenshot using xwud and take a screenshot of the latter:

• place the X window on the main screen (it does not work for me on dual-screen setups)
• Open an XQuartz terminal window
• xwd -root -out screenshot.xwd
• xwud -in screenshot.xwd
• now take a regular screenshot of the xwd visualizer with any screen capture software

This system avoids the need for other tools to be installed and works directly with XQuartz.

Go to Preferences under the XQuartz menu and then click on Output. You'll see a pulldown for colors. Make sure its set to millions of colors. Then restart XQuartz.