Why was the color of Ezio's robe changed in AC: Revelations?

The robes aren't blue, they're the original white, just worn out and stained with the grime of a year's worth of travel. Even with regular care and cleaning, a set of white clothes exposed to road dust and rain will stain enough that it becomes grey before long. And that's what colour his robes are at the beginning of AC: Revelations: grey.

The blue-ness you perceive can be explained though: the intro cinematic takes on a blue tone as soon as he arrives at Masyaf, making everything (even the back of the templar's bald head) tinged blue (jump to 5m43s if the link doesn't do it automatically):

But when Ezio, still wearing the same outfit, arrives by ship in Istanbul, you can see that in the colour-balanced light of sunlight free of dramatic tinting his robes are a travel-worn grey (same video, 11m20s):

Since Assassin's Creed II, it has been shown that Assassins don't always strictly wear white robes, some even wear no traditional robes at all, such as Ezio's uncle. It's not like it's a strict rule or anything.