If you aren't making money off of it or claiming that you created assets from scratch when you didn't, it should be fine. Recolors are a thing and some of them can be pretty popular! I think it's a bit different if you're using sprites from someone else's mod - in that case you need to ask permission. But when it comes to the vanilla assets, youre essentially making a fanwork for the game itself. Fanfics sometimes use real lines of dialog from the source text and imo altering existing sprites in a mod isn't all that different
The amount you modify can also vary a lot! Here's some examples from a mod I'm working on
Keeping mostly the same and just changing a little, we have Charlie the chicken being transformed into Charlie the duck
Then there's this, which was the art for the crab pot bundle, recolored, chopped up, and tweaked to look like the crayfish instead
And finally there's this clam, which I made from scratch, just using the actual clam sprite as a visual reference + color picker
Point is, there's a pretty wide range of what using the vanilla assets can be!