Rainbow Friends is a Roblox game. I do not use Roblox, and have not played Rainbow Friends, but a lot of kids I work with play it, so this is my brief research into it.
Rainbow Friends is a cooperative creepy cartoon game; it uses a simple 3D engine not too much upgraded (visually speaking) from those used 20 years ago, with simply drawn cartoon characters haunting a dark maze of random settings. It appears to use building atmospheric creepiness mild jump scares as the main hook, drawing on the popularity of Five Nights at Freddy's and Piggy. As Roblox consists of user-generated games, the artwork is naturally primitive, but still well done and quite creepy.
The backstory is that a class is on a field trip to an amusement park
when a mysterious creature diverts their bus down a side road, resulting in a crash near a biohazard site. The players pass out, reviving in some sort of strange facility. A voice over the intercom informs players that it needs them to help find its blocks, and the players are released into a dark maze haunted by shadowy creatures.
These creatures are the Rainbow Friends, named such because they are different colors. They all have slightly different behaviors, but they basically act like classic arcade video game monsters in their patterns of behaviors. We quickly realize that this is an old abandoned amusement park, and the rainbow friends are... sort sort of mascots, I guess? Players can find a lab with a machine that was apparently used for making the monsters (it's just a blue booth with the words "this is where they were made" scrawled on the wall; the designer kept things simple). Anyway, now they want to kill you, so run fast!
The players have to work together to find blocks and other materials that will allow the mad genius behind this place -- the red monster, named Fred -- to complete his mysterious plans. Then the blue monster will chase you out of the game, and you win.
I don't especially like the Roblox aesthetic, but this looks like a pretty good, and very spooky game. It also has a lot of clever clues to the backstory, giving users a mystery to solve if they want, but this can be easily ignored if you prefer to focus on the gameplay. I suspect I would have liked it quite a lot as a kid, and all the more so because of the cooperative nature. My brief research into the game did not convince me to sign up for Roblox, but YMMV.
It is generally believed that there will be more Rainbow Friends games in the future, with at least two more monster designs already known and many rumors flying around the younger parts of the internet. However, as of this time, the second chapter has not been officially announced.