x-display-grayscale-p

x-display-grayscale-p is a built-in function in `C source code'.

(x-display-grayscale-p &optional TERMINAL)

Return t if the X display supports shades of gray.
Note that color displays do support shades of gray.
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.