Its ok to use either as long as the texture on the sprite is primitive or made of primitive shapes, shading is fine as its just visual work but as long as the game is majority made of primitive shapes to give it a sphere, cube, triangle look then it falls under the theme fine :D

Numbers text and fonts can be anything you want, the more the game looks like its made from primitive shapes the closer you are to the target but dont restrict yourself on things like donts, text, etc, you can use art assets but that will take away from the basic primitive look of the game and will probably lose you points.

And it can be whatever colour you want, just keep primitive shapes in mind when designing your game!

THINK GEOMETRY DASH!!! :D