Heh, I've posted this info so often, I should get a commission :)
You can try using a colored grout sealant like the one from AquaMix.
If you call their 800 number, they'll send you a color sample chart,
and then you can order their products from an online retailer. You have
to be very careful when you seal the grout because the sealant is much
like paint; I used a craft brush and did a little at a time until it
was finished. Mine's been on my tile floors for a year and shows no
sign of wear.
The color I used (forget what it's called) dried exactly the color
shown on the color sample they sent.