If the tile is in great shape you might consider refinishing it as Joe said, as for adding new tile on top thats not something I would recomend.
I have tiled over existing floor tile, this can be safely done as long as the subfloor and existing tile are sound, on walls I would be concerned with the build up.
I had my bathroom tiled for the first time in an older home 35 yrs ago. I will never ever wish for a white bathroom again. Hardest thing in the world to keep clean. I can't tell you about how to do the tiling but reconsider your color choice.
We did this about six years ago and used a company out of Sugar Hill/Suwanee area called Unique Re-finishers. They came in and "re-glazed" the tile with an appoxy. It has held up extremely well and still looks great.
Good idsea? probably not
In my personal home I covered my wile wainscot with beadboard
I have tiled over existing floor tile, this can be safely done as long as the subfloor and existing tile are sound, on walls I would be concerned with the build up.