I'm not sure if this is what you mean, but both DS6 and I think of numbers in a rather purplish/gray color for the whole number and then when we're working with a particular digit it lights up yellow in our minds. One of the coolest teaching moments I've ever had with him was when I was teaching him about the number line and told him to close his eyes and imagine the numbers counting from one to the next and he says "Oh COOL!" and I asked him what he saw, he said "When I think of the number it lights up!" I hadn't told him but that's exactly how I see numbers, too. As far as mental math, he and I both have a "base", mine is 10, his is 2, no matter the problem I always look for a way to work with 10 he always looks for how to divide it by 2.
I didn't know there was any research about it, that's really interesting!
