I had the problem of Quake 2 being very dark under OpenGL. Searching this forum revealed I was not alone. I’ve found a solution which is simple to try and works for me.
Go to the Quake 2 console and type:
gl_modulate n
where n is 1 (the default), 2, 3, 4 or 5 (which I use).
To activate then type:
vid_restart
If this solves the problem change the command line in the Quake shortcut to:
c:\quake2\quake2 -gl_modulate 5
Cheers
Another Dave