Grouting Question?

I am looking to re-grout my bathroon - just moved in and its kinda grey in colour (not attractive) - do i need to chip away and remove the old grout before applying new stuff?

Or can i simply grout over to whiten things up?
Thanks

Answers:
I ran into the same problem when I purchased a home last year. Essentially, you have a couple of different options other than re-grouting - which is a lot of work. The first option - which I chose to use - is a grout re colorant. You can find the product at Lowe's and it actually worked really well. Just get a kids toothbrush, apply the product to the grout, let dry for 15 minutes and then wipe off anything that got on the tile with a wet sponge and re-seal. I'm going to install a tumbled travertine backsplash. Is it okay not to fill the natural holes in the stone?
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