The Rainbow Room was located in Hawkins National Laboratory. It served as a playroom for Eleven, Kali, and possibly other child-aged test subjects.
The room could be differentiated from the other rooms in the lab due to a rainbow mural in its doorway. Despite its purpose, the room itself was actually rather bleak, having plain white walls and hard white flooring. The only other object present in the room, besides a few toys and some lab equipment, was a white table.
In the early years of her life at the lab, Eleven would play in the rainbow room with Eight.
In an effort to retrieve her missing daughter Jane (Eleven), Terry Ives forced her way into Hawkins Lab, threatening the lab’s employees with a gun. She eventually made her way to the rainbow room, where she found Jane and Kali playing. However, she was caught by lab personnel before she could take Jane and escape.
According to Kali, one day she went to the rainbow room and discovered Eleven had been taken away.