A button that updates it's status to every viewer of a webpage?

This is my problem:

• I have a webpage hosted in a raspberry PI
• It is being viewed by 2 clients in different parts of the internet
• On the page there is a button that when clicked, should turn green
• If USER_A clicks it, I want both USER_A and USER_B to see it turning green "at the same time"

What tools should I use to accomplish this?

I mean that I don't want USER_B to refresh the page manually to see the effect.