Almost by definition if you have a full broadcast of flakes (broadcast "to rejection) the underlying epoxy is completely covered. Say pink epoxy, with a full broadcast of black flakes when finished you wouldn't see a peek of pink. In the real world not every falke adheres, and there will be spots where you'll see the epoxy, so use an epoxy color that is similar or works with the flakes.
To figure out how much flake is needed to rejection multiply the square footage by 0.15, i.e. 500 ft2 would need 75 lbs. of flakes. If the flakes are smaller than 1/4 inch you may want to pump that up 10%.
Color choice is pretty subjective, but with a bright car - like yellow - a darker floor will really set it off nicely, IF (and that's a big IF) you have a very good lighting system.
If you can't make up your mind, do what many of our clients here on Garage Journal do... ask the wife.