I suggest either:
A) do the old costume as originally planned, which matches the bodysuit you already have
B) buy a new suit in grey if you're now smitten with the new costume
C) Buy contrast fabrics and just make the suit
In order to paint a lighter color over black, you need to use an opaque fabric paint. You will have to wear the bodysuit as someone else paints it, and once the paint is on there, you will lose almost all elasticity in the fabric. It won't be able to contract to a snug fit against your body, and it will be stiffer. It won't be nearly as stretchable either, and if it stretches more, it'll possibly split the paint.
On the other hand, if you want to go from grey to black, you can use a transparent, flowable fabric paint like Dye-Na-Flow, which does not inhibit stretch. The "galaxy" effect can be painted, and while it would have to be done with an opaque paint, the coverage is not solid so it wouldn't affect stretch much at all. The one thing about Dye-Na-Flow is that it spreads and bleeds (it works just like a watercolor paint, after all), so to get sharp edges and control the bleed effect, you need to prepare your fabric with a resist or flow stopper.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.