I found this while wondering how to re-grout floor tiles, I'm in a rented house for the past 5 years,basically the tiles are here longer than me! dark smallish tiles in the kitchen with dark grout, but light tiles in the bathroom & I'm not convinced it was dark grout to begin! I would think a lighter colour grout would be a good thing if they are large tiles! It's the black tiles that are going to show the dirt more not the grout! Sounds silly but true.
But can you tell me is your grouting level with your tiles, bit above your tiles or somewhat below??