Double Rainbow

It8G21UEbwI Double Rainbow is an easter egg featured in Halo 4. The Double Rainbow can be found in a waterfall, on the campaign level Reclaimer.

Walkthrough
Around halfway through the campaign level Reclaimer, shortly after a Type-56 Lich explodes over the ocean, the player comes towards a small ravine with several waterfalls. Go stand on the nearby cliff on the edge of the ravine. As the Mammoth approaches the player, the first rainbow becomes visible in the ravine. As the Mammoth makes its way towards the ravine, the second rainbow appears in the ravine beside the first.