It is probably the X server. In dGB-GDI V2.8, you will by default be using the Qt library-based viewers which support all display modes. Releases 2.7 and lower, however, have only the viewers based upon the Seismic UNIX viewers. These are 8-bit (PseudoColor) only.

Even with the old viewers, we do work on TrueColor display, with the kde window manager. But, for displaying grids and variable density seismics, we then switch to a second (extra) X server. If you work on Linux, this is what you need to do:
A - Make sure the following setup items are OK:

- Virtual console support (Try if you can <Ctrl-Alt-F1> and <Ctrl-Alt-F7>)

- The old fvwm (fvwm1) or twm installed. You cannot use fvwm2, kde or other color-guzzling window managers

      #!/bin/csh
      setenv DISPLAY :1
      xinit .xinitrc.fvwm -- :1 -bpp 8 &