Fairly surprised this isn't on here, but I'll make a thread for it. A few months ago there was an hour long special planned to release this Summer. From what it looks like, it seems to it will air in the middle of Season 9 sometime in mid-June. The date that says it will premiere is June 29th, 2019.
At the moment, there isn't much known about the special but there are a few sources that at least says it exists.
Synopsis: "When Twilight Sparkle and her friends arrive in Hope Hollow, they find the town and all its residents completely devoid of color – their dream vacation is starting to feel more like a nightmare! The Mane 6 will have to solve the mystery behind this washed-out world if they hope to bring the "rainbow" back to the Rainbow Festival and color back to the lives of their new friends."'
I will personally update this thread on the goings on of this special since it seems to be coming fairly soon in the next couple of months.