How to run XQuartz in 16-bit color mode

Valid values for "depth" in defaults write org.macosforge.xquartz.X11 "depth" -int n where n is one of the following:

• From Display: -1
• 256 Colors: 8
• Thousands: 15
• Millions: 24

These values were obtained using defaults read org.macosforge.xquartz.X11 depth after selecting each of the options in the menu.

Here is the output of the defaults (read/write) and xwininfo -root commands:

$ defaults read org.macosforge.xquartz.X11 depth
24
$ defaults write org.macosforge.xquartz.X11 "depth" -int 15
$ defaults read org.macosforge.xquartz.X11 depth
15
$ xwininfo -root

xwininfo: Window id: 0xf1 (the root window) (has no name)

  Absolute upper-left X:  0
  Absolute upper-left Y:  0
  Relative upper-left X:  0
  Relative upper-left Y:  0
  Width: 2560
  Height: 1418
  Depth: 15
  Visual: 0x22
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x21 (installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +0+0  -0+0  -0-0  +0-0
  -geometry 2560x1418+0+0

$