It's this setting: https://github.com/aosp-mirror/platform_frameworks_base/blob/1cdfff555f4a21f71ccc978290e2e212e2f8b168/packages/SettingsProvider/src/android/provider/settings/backup/SecureSettings.java#L68. You can trace it in the source code. The general name is Reduce Bright Colors. Check Graphene's src for that. Woulda linked Google's AOSP web page but it's giving me 503 because that's how Google do these days.