No idea I'm afraid, but I think it looks grand as it is. Always disappointing when something so permanent doesn't turn out as you hoped/specified.

We had a small section of bricks repointed with the wrong colour mortar, but just dabbing the mortar with a small amount of watered down paint, until we came to approximately the right colour, was the answer. So watered down MATTE paint might possibly work. If you sponged it on in degrees and wiped it off the edges of the stone as you went? I'm not an interior designer though. Just a renovator. But that seems a less severe option than resorting to bleach.

Thanks, Helen. The pic makes the grout look light than it is.

But I agree with Helen Strand's comment - I think the grout looks good.

I have had this happen when I had my back turned on a renovation project ;-(. There are some solutions but they range in price (and I live in an expensive country):
1. You can have the grouting dug out a little to regrout it in your colour - labour is expensive and it is painstaking. But it can be done. $$$
2. I don't know where you are, but there are some products that you can use to recolour your grout . In this instance, talk to your tiler/contractor and ask them. They are still pretty iffy (as far as I am concerned) but each day, new and better products are emerging.
I am passing this discussion onto Ironwood (one of our Houzzers) who probably has a lot more background and helpful information than I
Best of luck.

http://www.wikihow.com/Regrout-Tile

You can try - good luck! :)))

That type of stone usually has a bit more room than most which can be regrouted with a lighter color. Grout on stone is usually done with a piping bag (like decorating a cake), not with a grout float. The matte paint could do the job too.

Thanks AVM! Two things...maybe more if I get going good. All mortar will fade over time. The dye is not oxidation proof...but now is when you hate it, so time is not a friend. The only really certain way to change the color is to grind out the mortar and re-point the joints. A mini-grinder outfitted with a diamond wheel in the hands of a trained professional. Then a grout bag and a properly vetted recipe for the correct color mortar. I have a hard time thinking about trying to do a liquid die or paint on the grout only. This isn't a ceramic or porcelain tile, The chances of straying outside the lines are a lot more consequential than my grandson's coloring book... One possible alternative. I did a massive faux finish job once on an interior (formerly exterior) limestone wall. 100' wide by 110' tall. We were tasked with getting rid of power sprayer wand marks all over the wall. We had to fool people that nothing bad had ever happened...so we took limestone powder and powder puffed each 2'X3' stone, wiped it, ragged it, added veining with a set of wax crayons, then wet it and dried it and powdered it again. It was very much "lipstick"...but we cost less than $30K to make it look good and took a month. The alternative was sandblasting and that was over $100K and would stop work on the floor below. Dang...I was hoping I wouldn't get going.

Thanks for all these really helpful comments. It's been over 10 years since this was done and I guess it still bothers me. But not too too much. I was just wondering if there were any options and now I found out!

https://www.chrislovesjulia.com/how-to-diy-over-grouted-stone-fireplace-for-under-200/