Can basically echo what the reviews are saying, personally. I think the 8-9.5 reviews are pushing it, though. I'd rate it a 7/10 (for reference, my other Paper Mario game opinions: 64 is 9/10, TTYD is 10/10, SPM is a 6.5/10, and Sticker Star is a 3/10). Rounding down a bit, it was about a 20 hour playthrough.
Ditch the combat and the game leaps to around an 8.5-9 range for me. The combat is just awful, the only saving grace is that action commands in Mario RPGs are always pretty fun to use... but what's the point when the battles themselves are clunky and largely a waste of time? Someone linked the Arlo review, and I'd recommend giving that a watch to see a demonstration on just how slow the combat is. He basically nails every issue with it (and later goes on to detail other issues with it), the only criticism I'd lob at him is that he could've really driven the point home by showing that even Sticker Star's battles were a significantly smoother affair. Along with that, Sticker Star had significantly fewer mandatory battles, whereas Color Splash loves to toss you into mandatory fights.
Color Splash is undeniably better than Sticker Star in every way (except for the battle system). It's an incredibly charming game that in its best moments brings to mind classical adventure games. That's really fundamentally what this game is. Ironically, its biggest problem is pretending that it's still an RPG. Cut the battle system, vary up the NPCs (even if they're all Toads, give them interesting designs and names at least!), and fix up a few poorer quality segments, and then you basically fix every problem in the game. As it is though, it's a pretty excellent take on Paper Mario as a Lucas Arts game. It's a flawed game, but it's a very good game. This is up there with Doom as one of my biggest/most pleasant surprises of 2016.