I would remove all of it now and start over use something like Fusion Pro, that comes pre mixed, so no issues with color variations. It is also stain resistant and has Microban. It does require a bit more effort to install as you need to work in smaller sections.

http://www.custombuildingproducts.com/products/grout-materials/single-component-grout/fusion-pro.aspx

We have the same issue but it's not that noticable. I didn't even point it out to dh because it is relatively minor.

If there were big variances I'd be trying the easy way first.

As long as it's not in the shower you can fix it, I have before. In one new house the final cleaner spilled a cup of coffee on the floor and it would not clean out of the grout. I bought a small jar of Grey acrylic paint, diluted it, used a small foam brush and skimmed over all the grout. It turned out great and lasted. The more you dilute it the lighter color it will be. I'd start light first to see if that works. With it thin, it will soak right into the grout then you could seal the grout. It's not a hard job at all.....

MiMi - That's encouraging. Why do you say as long as it's not in the shower I can fix it? This is a new bath with shower, and the grout is inconsistent everywhere (floor and shower walls).

I think Mimi is saying if you go the acrylic paint route it needs to be in a dry area. Also not sure how it holds up to traditional cleaning methods

I don't believe actual grout colorant has the same requirements.

Sale , would you mind providing an update if you decided to try changing the color? Thank you!