I can't vouch for either of these suggestions, but there was a recent post about this that offered two tips:
You can contact Grout Shield, a member hf hometalk. They carry many different shades of grout, and I believe can match what you are looking for if they don't have it, by sending them a color swatch.
Thanks Terri and Steve. This is very helpful
I use concrete colorant to change the color of the grout. I did a granite tile countertop in absolute black and wanted black grout, all I could find was a dark gray so added the black colorant - they sell it at HD and Lowes and most home stores that carry concrete mix.