In a 2011 interview with Jeramy Cooke, art director for Borderlands 2, explains the process of designing new, more colorful environments for the then-upcoming game in the Borderlands series.
In the original Borderlands, we got to play in the badlands which were very brown. With Borderlands 2, Gearbox Software is bringing more color to the game.
This interview goes over the process of creating new those new environments but still keeping the old Mad Max feel the first game had.