emm … if you asking if it is posible , Yes it is .
You can use socket.io , it is a librery to send and listen data in “real time” .
In you page you can listen if other user change the color and you can send it to change , and in a few milisecons, if your configurations and code are correct , you will be able to change the color of the background for all users
More information here
solved Connect two people to same server?