With spray-paint, it’s super easy. Just spray the cups before you start building. Make sure the spray-paint is made for plastic, then go to town.
I like to spray the thickest coat on the cup bottoms, with just a little overspray on the sides. Once the ball is together, the wires are hidden but there’s plenty of clear plastic to shine and reflect the light. A little like a tie-dyed effect. To do this, spray once the halves are built before you add the lights.
You can also use more than one color to make a pattern. I generally don’t paint the insides of the cups, just the outsides, but you can paint a sparkleball any way you want :)