Manually specifying the Visual for a GTK+ app?

[jonathan claggett philips com]

Hello GTK developers,

I am developing a GTK+ application via the Perl bindings to be run 
alongside a 'legacy' application. I use the term legacy because the 
ancient thing requires that the default Visual be 8-bits and then proceeds 
to XAlloc most of the available 256 color slots. The good news is that the 
video card I'm using supports multiple visuals including GTK+'s favorite: 
TrueColor. Thus my question: Is it possible to initialize a GTK+ app to 
use the non-default 24-bit TrueColor visual that is available?

I'm aware of the GDK Visual functions like get_best() but I've no idea how 
to use them to specify the visual for e.g., a MessageDialog in my program. 
Ideally, I'd like to be able to specify up front what visual should be 
used for all ensuing GTK+ widgets.

Thanks in advance for any and all suggestions,
Jonathan

Here is an excerpt from xdpyinfo's output on my machine:
...
screen #0:
  dimensions:    1280x1024 pixels (361x288 millimeters)
  resolution:    90x90 dots per inch
  depths (3):    1, 24, 8
  root window id:    0x35
  depth of root window:    8 planes
  number of colormaps:    minimum 1, maximum 1
  default colormap:    0x22
  default number of colormap cells:    256
  preallocated pixels:    black 0, white 1
  options:    backing-store YES, save-unders YES
  largest cursor:    unlimited
  current input event mask:    0x38003f
    KeyPressMask             KeyReleaseMask           ButtonPressMask  
    ButtonReleaseMask        EnterWindowMask          LeaveWindowMask  
    SubstructureNotifyMask   SubstructureRedirectMask FocusChangeMask  
  number of visuals:    7
  default visual id:  0x24
  visual:
    visual id:    0x23                                  <---- This is the 
one I want! -------<<
    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
  visual:
    visual id:    0x24                                  <---- This is the 
one GTK+ sees! ----<<
    class:    PseudoColor
    depth:    8 planes
    available colormap entries:    256
    red, green, blue masks:    0x0, 0x0, 0x0
    significant bits in color specification:    8 bits
...

And, for simplicity's sake, assume my my perl-gtk application looks like 
this:

#!/usr/local/bin/perl

use Gtk2 '-init';    # The '-init' means Gtk2->init gets called initially
Gtk2::MessageDialog
      -> new($no_parent, 'modal', 'info', 'ok', 'This is only a test')
      -> run();