My understanding of a flat UI design would not be to grey out icons but to 'hide' what isn't relevant to the user, leaving a more clean and minimalistic interface.
However, one of our software developers wants me to design a flat UI with 'greyed out' buttons (and probably other things too, but we haven't got to that yet).
I'm just wondering if that's something that you would do with a flat UI, and if so, how that's possible?