What you're describing is a toddler alarm. MAde for children that can't yet tell the time, but you put in the room and they show one color for sleeping and another for awake. Look up "toddler alarm" or "ok-to-wake clock" Here's the first one I found via google (non-affiliate link)
https://www.amazon.com/Hatch-Baby-Night-Light-Machine/dp/B06...
Along the path, we got woken up at 4:46 (there’s a 6!) and once to “Mommy, Mommy, it’s not 6 yet in your room either!” as he excitedly learned that time was the same everywhere.
Kids are exhaustingly awesome!
This reminds me of the following:
Imagine you had some items, and you gave each item a unique name, and then used the unique name you gave to the last item to describe their amount.
That's what numbers are, a commonly agreed upon way to label items so the last label indicates how many you have.
But you wrote "there is literally no other value in time except the fact that it's the same everywhere". This is wrong.
15 minutes before my phone alarm goes off (not configurable sadly), the whole screen comes on a very dim red, and slowly fades through medium orange to a bright yellow at the time of the alarm.
With the phone laying flat on top of my dresser, this provides a great lighting cue.
I like the idea though... I think I've got an RGB light strip I pulled out of a pc case (not a fan of rainbow vomit styling...) around here somewhere. I'll put it on my growing pile of projects I should probably do.
Turns out it's not really the color that is the most important factor for wakefulness-shifting, but the amount of light your eyes perceive. Yes, the cells responsible are more sensitive to bluish light, but a ton of not-so-bluish light would keep you awake just the same, and just a little blue light wouldn't be that harmful.
That being said, 'blue for Go' still makes more sense to me instinctively :D