So I have this theory. It's based on one of the interviews I read in the SPN mag this month.
In it, whoever was being interviewed said something along the lines of demons being the result of mixing a human soul with angelic power. So that says to me that all the demons powers, their juice, comes from Lucifer (and maybe the other angels that fell with him? Or did they just get their Grace yanked and born as humans? What happened to that 1/3 of the host of heaven?).
Now, your run of the mill, average demon (with the wide spread and variety of their powers notwithstanding), has black eyes. More powerful demons get to have different colored eyes. The original crossroads demon that we meet has red eyes. Azazel's eyes are yellow. Lillith and Alistair's eyes are white.
My theory, such as it is, is that the closer the demon gets to the original source, the more powerful they become, the more 'angelic' in power level they hit, their eyes lighten in color. Given that Lucifer, outside of a host, is (as far as we can see) a bright, bright light (I saw a fic once, where Cas described himself as a highly intelligent sunbeam in his true form - sarcastically, of course), this makes some sense to me.
Have we ever seen Crowley's demon eyes? I don't remember. But I want them to be not-black. Because, if he's essentially, by default, in charge of Hell (everyone bigger and badder than him appears to be dead), then he should be fairly badass.