Typically, lighter grout means too much water used during cleanup. Sounds like you've already confessed to the crime.
Most cases, the grout color underneath the surface is correct, so you just have to remove some of the surface material. Use vinegar and a stiff nylon brush, scrub the grout until you see a change of color, then rinse off with clean water. It may take a couple or more scrubbings to get the right color.