The grout is probably a dark gray, not white. If it's clean you can use an epoxy grout colorant to both seal and color the grout. You ought to think about doing a redo in a few years. If you use muriatic acid, that redo will be much sooner than later. It can and will dissolve grout and if a tile has crazing, it can get into the interior and ruin it as well.

I posted this previously....again, cannot vouch for the method!

"I just read this posting so I cannot vouch for the hint concerning safety for grout and tile. There are before and after pictures using Woolite Carpet Foaming cleaner:

http://www.aberrysweetlife.com/2013/05/cleaning-tile-grout-with-carpet-cleaner.html