display-visual-class

display-visual-class is a compiled Lisp function in `frame.el'.

(display-visual-class &optional DISPLAY)

Return the visual class of DISPLAY.
The value is one of the symbols `static-gray', `gray-scale',
`static-color', `pseudo-color', `true-color', or `direct-color'.
DISPLAY can be a display name or a frame.
If DISPLAY is omitted or nil, it defaults to the selected frame's display.