Grout Dye

[SJTD]

So the new grout is too light. Any proven ideas on making it dark grey?

The stuff the major grout makers sell is paint marketed as dye. Wears and may peel.

[230ohc]

There is a stain powder for mixing in when wet, but I don't know of an after the fact oops eraser.
You might try making a dye from the stain.

Concrete dye, stain.

If you have an impregnated grout your efforts may be lack luster as the impregnator make the grout less soluble and therefore stain resistant