TL;DR; As Paper Mario games, there both absolute monstrosities. They're nothing like the first three games, and if I were you, I definitely wouldn't worry about skipping them, even if you plan to play PM The Origami King. Arguably though, in my opinion Color Splash is still worth playing if you have nothing better to do. As games in general: Sticker Star is one of my most hated games of all time (out of the ones that try to be a good game at least). The story is pretty much this: Shiny thing falls from the sky, Bowser touches it, he becomes OP, The pieces of the shiny thing scatter across the world, and Mario needs to collect all of them to defeat Bowser; The End. I'm fine with people who like the other aspects about Sticker Star; I've heard that some people like the music in the game. If I had never played Sticker Star (partially) then I'd probably think that it's ok, but nothing special, but now Sticker Star's soundtrack is permanently attached to that game for me, and I sadly just hate it. Color Splash on the other hand, is an enjoyable game to me at least, just it's more of a turn based Mario spin-off than a Paper Mario game. I don't have too much more to say about Color Splash, it's just a bland game. Why have my replies been so long recently??? Now that I'm out of school for quarantine, I think that I'm actually thinking more... Online school can go die though.

Pretty sure the school system relies on the fact that students aren't likely to think about what's happening, teachers have no idea what to do when I question the system

Sticker Star is OK, to be honest. It's just that when the main villain is Bowser, the game lacks originality. That's why SPM was awesome! Original Villain (Count Bleck) and a great plot. Sticker Star's plot was paper thin. Colour Splash was fun. Better than Sticker Star. It's no SPM but it is still an OK game, as any Paper Mario game is! I never played Paper Mario 64 or TTYD, but I hear they were good. Paper Mario: The Origami King is definitely going to be worth the time though! It looks great!