Some of the people didn't seem to get it, so I commented:

THE 'GREEN' BALLOON IS YELLOW. IT TURNS GREEN WHEN THE GREEN LASER HITS IT. SINCE YELLOW IS HALF OF WHAT MAKES UP GREEN, MUCH (HALF?) OF THE LASER ENERGY IS RE-EMITTED AS VISIBLE LIGHT. THE ORANGE BALLOON CONTAINS RED, WHICH PERFECTLY ABSORBS THE LASER ENERGY, BEING THE CHROMATIC INVERSE OF GREEN.