This image is a screen cap taken from the D3 Character and monster panel, which focused primarily on the Monk. The animation was a joke, playing on the ongoing “too colorful/no-rainbows!” art controversy. As you can see, one of the animators had added a rainbow streak to the Monk’s roundhouse kick, as shown in some sample footage from their Matrix-like test program.
At least we assume it’s a joke?
Incidentally, I did see one rainbow in the Blizzcon build. It was way back in the cliffs and rocks in the mining pit area of The Borderlands, and almost certainly included as an Easter Egg/inside joke, since there was no trace of surface water in any area shown in the Blizzcon demo areas.
Thanks to a reader for uploading this image, which reminded me that I’d meant to post about the rainbow kick in the first place.