Zwift added a brightness auto-adjust feature in its February 1 update. The official release notes explain it this way:

Graphics brightness auto-adjusts now so that caves, subways, valleys in shadow, etc are much brighter than before

But some Zwifters are complaining that the feature goes too far, resulting in a “washed out” look in these previously dark areas. Here is an example to see how this looks on my Windows PC, first without auto-brightness, then with:

Astute Zwifter Al Clewly sent along a tip he received from Zwift support to disable the auto-brightness feature. Simply add this “<PERF>67108864</PERF>” to the CONFIG section of your Zwift prefs.xml file, which is located in My Documents/Zwift

With Zwift not running, open the prefs.xml in a text editor like Notepad. The beginning of the file will look something like this:

<?xml version="1.0"?>
<ZWIFT>
    <SPORT>CYCLING</SPORT>
    <CONFIG>
        <ANNOUNCER_VOL>1</ANNOUNCER_VOL>
        <MINIMAPZOOMPREF>2</MINIMAPZOOMPREF>
        ...

Simply add the line in bold below and save your changes. Start up Zwift, and auto-brightness will no longer be enabled.

<?xml version="1.0"?>
<ZWIFT>
    <SPORT>CYCLING</SPORT>
    <CONFIG>
        <PERF>67108864</PERF>
        <ANNOUNCER_VOL>1</ANNOUNCER_VOL>
        <MINIMAPZOOMPREF>2</MINIMAPZOOMPREF>
        ...

Disclaimer: editing your prefs.xml file incorrectly may result in problems running Zwift, so perform this hack at your own risk.

Enjoy, Zwifters!