Yes! It's clear the consensus is that he needs art direction to get consistency, but this is how you actually put that into action. He needs style guides 100%. It's also potentially a good longer term investment, you can re-use and adapt them.
The guide also needs to include lighting, which he has diddly-squat of.
That palette swapping trick looks like a great idea for indie games!
Re:Why All of Our Games Look Like Crap
by aardshark on Thursday August 22, 2019 @11:42AM
(#20769328)
It was used on the Gameboy Color to colorize the grayshaded Gameboy games.
You can choose what palette to use by pressing a combination of buttons on most start screens. See this example of Super Mario Land: http://i.imgur.com/HupBY.png
Then swapping out artists will be easier and the game will look more consistent.