That's how I came up with it, made it open source, recently passed 100 users over chrome webstore and it have 5-star rating as of now.
and yes you might say that why would I need it when I can do this with filters that can be toggled via settings, thing is I didn't wanted to greyscale my work sites as well and thus I made site exclusion, also have scheduler, intensity control and above that all it supports all the browsers including Firefox providing same experience.
Would love any kind of feedback over this!!
* https://support.apple.com/en-jo/guide/mac-help/mchlp2975/mac
* https://discussions.apple.com/thread/252274205?sortBy=rank
and
androids do usually have a greyscale mode on the devices preinstalled, I can work around a bit to make it compatible with firefox android as well but I think an app is what you are looking for? greyscaling particular apps right?