VTurnOnTruecolor - turns the trucolor mode on.
  VTurnOnTruecolor turns the truecolor mode on. What this involves depends on
the visual class. 
  If the visual class is PseudoColor, VTurnOnTruecolor backs the colormap 
contents and reserved colors to a temporary file. VTurnOnTruecolor also 
updates the colormap according to the bit planes divided into red bits in the 
left most (most significant), green bits in the middle, and blue bits in the 
right most(least significant). If the number of bit planes is 8, then red bits
occupy the most left 3 bits, green bits occupy the next 3 bits and blue bits 
occupy the most right 2 bits. VTurnOnTruecolor uses a fixed colormap according
to the above method by dividing RGB into 3-3-2, then finds the closest color 
and colorcell from the fixed truecolor colormap for the reserved colors. 
  VTurnOnTruecolor resets all background and foreground colors for all windows
and redraws the contents of the button window, dialog window, title window and
menu. VDisplayDialogMessage should be called to redraw dialog window messages.
If the visual class is GrayScale, VTurnOnTruecolor returns an error code. 
  If the visual class is DirectColor and truecolor is off, VTurnOnTruecolor 
backs up the colormap content and updates the colormap to be identity 
  VTurnOnTruecolor does not set the overlays if the visual class is 
PseudoColor, but sets it if the visual class is DirectColor. VTurnOnTruecolor 
should be called for a PseudoColor or DirectColor visual class, if 
VDisplayColorImage is used to display a color image in the image window or one
of its subwindows. When the truecolor is on, the colormap cannot be modified.
Return Value
  0 - work successfully.
  1 - memory allocation error.
  220 - truecolor option is already on.
  222 - try to turn truecolor on in GrayScale visual class.
Side Effects
  Overlays will not be available if visual calss is PseudoColor. Any
message in the dialog window may have to be redisplayed.
Entry Conditions
  If VCreateColormap is not called earlier VTurnOnTruecolor prints an
error message to the standard error stream, produces a core dump file and
exits from the current process.
Related Functions
VDisplayColorImage, VTurnOffTruecolor.