I've been wanting this feature too.
I have used an Android Huawei watch and this has that feature. When you want it there is a full featured colour display, then when after a minute or so it changes to a basic black and white image for time only.
Presume that is an OLED screen so black is effectively off.
Is the Apple screen OLED too?
The Huawei watch lasts about 24-36hrs if not heavily used so it is certainly doable.
It isn't just for convenience but just think it looks nicer than having a completely blank square/circle on your wrist!