If it's lighter because you used a little to much water you may have efflorescence (white-ish looking stuff that leeches to the surface) that can be cleaned off.
On the other hand, portland cement can be unmercifully unpredictable, and colors in a store, on a chart, under florescent lighting is very unlikely to be the real world color in your finished job. If this is the case you only practical option is a grout stain. Bostic makes an oil based stain that is very good, I'd avoid the latex versions (based on personal experience)
Technically, you could remove the grout and start over, but trust me, you don't want to.