I had many of the bluestones in my patio reset and re-grouted by a
mason. Now the new dry grout (made with portland cement) is a very
different shade of grey than the non-replaced grout which has been
there for upwards of 30 years.

Is there anything I can purchase at Lowes/HD that can soak into the
grout to make it a more uniform color/shade?

Thanks,
Walter