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