Despite the datamined colour being a false alarm, there's still a pattern that suggests CP11 will have a purple background:
On this 3x4 table of all the DLC fighters on the
official website, characters vertically opposite each other have strikingly similar background colours, leaving Hero's purple without a FP2 counterpart yet. As dumb as this trend sounds, and as much as I'd normally dismiss it, people have noticed it since Steve's reveal and have seen it become a more legitimate possibility.
It also helps that a few of the more heavily discussed characters in speculation nowadays (namely Nahobino, Frisk and Octoling) would fit a purple background quite well. As stated before, however, the choices of background colour for the last three characters don't correspond too much to the characters themselves or their designs, and it would've been extremely hard to predict these characters based on only their background colours.
Side note: I was going to jokingly censor the name Nahobino as if it were a swear word, but that would've looked too similar to a certain racist slur.