I'd be pretty surprised if it can actually do a true according-to-Hoyle 256 grays. Until recently there were probably more laptops shipped with 18 bit (IE, six bit per channel, IE, "64 greys") LCDs than 24 bit displays. Someone even tried to sue Apple over it..

Not that it really matters. The human eye has to work to tell the difference between six and eight bit grey, and even 4 bit grey can look pretty good. (far better than 4 bit color.)