My bay has black eyelashes and Mum's creamy pali has really pale cream eyelashes so it works for our guys. Some grey foals, the first you can tell of grey they have white eyelashes, I think that was the application the person was talking about (it was a debate over grey or not grey, I think).
We will have to see. I have seen/heard of foals that stayed real creamy until they were 2 and then were REALLY dark sooty by 3. Kind of defeats your theory. I don't think there's really any definite way to tell. Just as long as she stays looking like a buckskin, I'm happy.