X -query `hostname` -bpp 8 -terminate :8
to start another X-session with 8 colour planes (the -bpp 8 parameter) which you can login to and run IDL in. You can switch between your X-sessions with Ctrl Alt Fx where x is in the range 6-10. This advice applies both when running IDL remotely, and when running your own, local copy of IDL---in both cases it is the bit-depth of your local X display that matters.
A similar method may indeed be used to obtain a full login session from one of the NBIfAFG servers. For example, to start a login session at ursa.astro.ku.dk, do the Ctrl-Alt-F1 and type
X -query ursa.astro.ku.dk -bpp 8 :1