nman976,
I was waiting to see if Adam had more info for ya, but I'll throw in an answer for what they do...
Mostly liners just add a little extra contrast to the primary handle material for looks. Some people will get a bright colored liner so if they drop the knife, they won't lose it, but don't want the entire handle that flashy. Sometimes liners are added to give more width to a handle for those users with big hands. Sometimes the liners are added to certain handle materials because they only come in thin veneers (i.e. - Muskox Horn). I like liners on some knives and not on others... depends on the style and materials.
As for colors, I think you can get most any color you like if you look hard enough, but I'm not sure which colors Adam offers. It would be pretty cost prohibitive to try and stock every color, but most makers will order a special color if asked (just be prepared to pay and slight upcharge for the additional effort).
Hope this helps and maybe Adam will jump in here to make any corrections to what I posted.
All the Best,
Stump