On Sun, 3 Jan 2010 07:58:52 -0800 (PST), infiniteMPG
wrote:

We have a kitchen floor done new with 12" X 12" tile and we're really
happy with the work with one exception, the grout lines. We're
finding we didn't leave the grout line high enough between the tiles
and the edges of the tiles aren't comfortable to walk on. We had used
a grout sealer/stain product called NuGrout on top of the grout to
seal and also color the grout to the color shown on the ad (it turned
out looking gray and was supposed to be light tan). We were wondering
if we can add some grout over top of the old grout to raise the
level. We don't want to get too aggressive with cleaning or priming
the old grout and possible damage the tile face so we thought someone
might have some suggestions.

All help would be greatly appreciated.
See http://www.groutshields.com/ I put this product down on top of my
previously sealed grout such as y'al explained above, changed the
color at the same time and it looks great.
so far I have the kitchen, bath and a small hallway done - no problems
after 6 months.