I think you can darken the grout somehow. Hopefully, you haven't sealed it yet. (Or it doesn't have a sealer in it because this may prevent the enhancer from taking.)
2 things: Check with the store you bought the grout from. They may have some insight. If not, or the grout has sealer thus preventing you from altering its color, perhaps it's not to worry about anyway. In due time, the grout will probably darken naturally, I mean with dirt and wear-and-tear. Of course, do keep your floor clean as you normally would but this may be moot in a year or so.