Blizzard rep, August 4th wrote:

If you want to blame anyone for the rainbows, you’d blame our art director Brian. Because Brian is more the person who drove the art style into where it is.

Blizzard rep, August 7th wrote:

Regarding Brian, he recently resigned to form a startup technology company (outside the game industry), which is why we’ve posted about the open position. This change won’t impact the game…we’re really pleased with the look and feel that Brian helped create for Diablo III, and the new person we bring onboard will work with the other artists on the team to maintain the art style moving forward.