Here's the basic gist: The team takes a few screenshots of the area in question, and then edits them with photo editing software. The changes made are then coded into a texture, which is placed on the area originally screenshotted. This allows the game engine to make the color adjustments on the fly.
According to the dev blog, this new tech allows the art team to adjust SWTOR's final color output in very much the same way one would edit a photograph, with almost the same amount of flexibility.
The blog shows off some cool before/after gifs of specific areas, so if you're into technical, under-the-hood things it may be worth a look.