How to determine number of colours (bit per pixel) in X11

Question

How can I see how many bits per pixel are actually available in a running X11 session?

Answer 1

You could use xdpyinfo:

screen #0:
  dimensions:    1366x768 pixels (361x203 millimeters)
  resolution:    96x96 dots per inch
  depths (7):    24, 1, 4, 8, 15, 16, 32
  root window id:    0x2b9
  depth of root window:    24 planes
  number of colormaps:    minimum 1, maximum 1
  default colormap:    0x20
  default number of colormap cells:    256
  preallocated pixels:    black 0, white 16777215
  options:    backing-store NO, save-unders NO
  largest cursor:    64x64
  current input event mask:    0xda4033
    KeyPressMask             KeyReleaseMask           EnterWindowMask          
    LeaveWindowMask          KeymapStateMask          StructureNotifyMask      
    SubstructureNotifyMask   SubstructureRedirectMask PropertyChangeMask       
    ColormapChangeMask       
  number of visuals:    240
  default visual id:  0x21
  visual:
    visual id:    0x21
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff0000, 0xff00, 0xff
    significant bits in color specification:    8 bits