if you're dual-layering transluscent prints like most shops do, two layers of black is opaque. if it were me, the grey's in my "base-layer" would be a bit lighter (if not borderline white) so that the toplayer's grey transluces....I might even add a bit of design to either the yellow text or the black background, like a honeycomb pattern, just for funsies.
p.s. Outlines are cool, but its not really "design" when overused.