Orange is especially selected for ping pong ball color precisely because it is the easiest to see, particularly in contrast to green (table) and gray/white (background). Think orange cones placed behind your wrecked-car-parked-on-the-shoulder-of-the-road. Think orange hunting vests, biking outfits, toy gun identifiers, carrots, your brother turning into a carrot (if he wasn't one already).
 

Hmm...now that you've reminded me, my brother DID turn into a carrot once...you folks should talk to me mom, and she can tell you the most amusing story....

Anyway, Carlos, stop eating so many carrots! Come to think of it, I think you shouldn't get anywhere close to orange ping-pong balls yourself...you know, lest you end up confusing them.......heh, too bad you didn't glow in the dark when you turned orange that time, that would've been cool!

;)

TQM!
 

I would vote for heat-seeking ping pong balls.

As lore has it, eating carrots is supposedly very good for your vision. Also eating enough carrots to turn yourself into a walking root is apparently very good for a laugh or three. But you just can't consume enough carrots to have good color vision.

Having inherited the pleasure of deuteranomaly, it greatly diminishes the coolness factor of green and orange for me. Neon-yellow-skinned ping pong balls would attract the most attention, I think. That color is a bit easier to spot.

Now heat-seeking neon-yellow ping pong balls would create for quite a challenging game, eh?

1 - You'd have to hit fast enough so that the ball just wouldn't start flying back at your face (assuming it wouldn't stick to your hand in the first place).
2 - Receiving the ball would require you to deftly sidestep or duck before you return (or get hit).

Okay, how about just a ping-pong ball with an inertially powered LED in the center, making it glow when active? I would consider BMW-Headlight Xenon Blue (circa last couple of years) or Bluetooth LED Blue. Easy to spot for those with most types of color perceptions :-).