actually a wireframe can be any colour it likes.
its point is about layout yes, but not necessarily graphics.
graphics "colour" a wireframe just as much as "colour" can.
and for it to be worth anything, its best if its coded, not a psd.
that way you work out the wireframe,
then you know what size components to add if and when you need to go to photoshop at all
this is in my opinion a black and white mockup of the site.