Well, since Tahaan and Aysus can "See" one another, and they both have been described with color aspects(Tahaan being described as Gray with little sparkles in in for example), I would say that "Color" is the word Tahaan is using to describe the way the shift in energy makes the bits "look" different. The change in the exterior aspect of the bit. I can't think of a better word. . . . .Sorry. . . . .

Or one could argue that a side effect of making Light and Dark, seeing as light consists of all the colors, is that color became a part of their vocabulary of describing the things they "See"