When priming a green (bathroom) sheet rock for the first time, is it important to block all traces of green visible through the primer? If I can still see green, should I apply a second coat of primer?
You can hide the color with paint or you can hide the color with primer. In general, primer is cheaper than paint so on "color changes" you want to do as much as you can with primer - even to the point of tinting your primer if you're not using white paint.
So, I would go ahead and use another coat of primer and then paint over it with the final color. You could cover directly with paint and still get good results, so it's really your call if the paint and primer are close to the same cost.