Here is the ASBC standard for grey:
Gray as in Gray SILKIE Plumage
Color of Male
HEAD: Dark Gray
NECK: Hackle-light gray streaked with darker gray
BACK: Even shade of chinchilla gray
SADDLE: To match hackle
TAIL: Main Tail, Sickles & Coverts-same as back
WINGS: Shoulders, fronts, bows & coverts-same as back. Primaries-slaty gray with center shafting of darker shade. Secondaries-slaty gray
BREAST: Light gray
BODY & STERN: Same as back
SHANKS & TOES: Thigh, shank and toe feathers-same as back
UNDERCOLOR: see female
Color of Female
HEAD: Chinchilla gray
NECK: Hackle-light gray streaked with darker gray
BACK & CUSHION: An even shade of chinchilla gray
TAIL: Main tail & coverts-same as back
WINGS: Shoulders, fronts, bows & coverts-same as back. Primaries and secondaries-solid even shade of slaty gray
BREAST: Light gray
BODY & STERN: Same as back
SHANKS & TOES: Thighs, shanks and toe feathers-same as back
UNDERCOLOR: Smoky gray of a shade not darker than top color