If you want to be shading the part you can't see, then for consistency you need to shade the whole damn board :)
Colouring in the bit you can see (as you've been doing) makes more sense to me.
Note that although stars orbit at the same speed, that doesn't mean they rotate at the same angular velocity, so stars closer to the centre still have shorter orbital periods than stars further out.
Also, the spiral pattern is generally considered to be a density wave. Stars and gas actually...