Hi Brian
Whenever using a color enhancer to try to darken grout keep a few things in mind. It's always possible that you won't get even results (no matter what manufacturer you use). You didn't mentioned whether or not you are attempting to darken sanded or unsanded grout. You also didn't mention what type of material is abutting the grout.
Sometimes it is better to do some type of acidic wash first, as this may cure the light grout problem. It may also even out the top layer of the grout (depending on your original wash technique). If your grout is surrounded by marble or another calcite based stone, then acid washing is not suggested.
Lastly, Enrich n Seal has a high solids content and could put a sheen on your tile surface (if not carefully applied).
Most important as Dave suggested, TEST first!