I’m not sure if this is a Google Android Pixel specific feature, but on my phone it’s under: Settings → ‘digital wellbeing and parental controls’ → bedtime mode → customize → grayscale. And then you set the timing by setting the bedtime timing.
Update: Looks like it’s a general Android feature: https://www.androidpolice.com/how-to-set-up-digital-wellbeing-on-your-android-phone/
I’m not sure if this is a Google Android Pixel specific feature, but on my phone it’s under: Settings → ‘digital wellbeing and parental controls’ → bedtime mode → customize → grayscale. And then you set the timing by setting the bedtime timing.
Update: Looks like it’s a general Android feature: https://www.androidpolice.com/how-to-set-up-digital-wellbeing-on-your-android-phone/