LibGGI guarantees that there will be a default palette when a palettized mode is set. What this default is, however, is dependent on the visual. For example, the X target deliberately avoids setting all colors to avoid color-flashing when moving between windows.
Applications that want to ensure that they have a full scale of all colors can call ggiSetColorfulPalette after mode set. This function uses a smarter color allocation scheme, causing good colors but still minimal flashing in windowed targets.