Hey! Good luck with your project - I believe in you!
To answer your question - yes 3d is okay. Any type of graphics style you want to use (as long as its appropriate XD) . The color pallet is optional and even with 3d art you can use those colors to inspire your art. For example, if you are 3d modeling some leaves or trees you can use the green colors from the pallet as the base color for the model. You can always use other colors - the pallet is there to be a jumping off point <3