https://buechelstone.com/how-to-choose-the-perfect-mortar-joint-and-color-to-complete-your-natural-stone-vision/

I would imagine that stone requires MORTAR not grout.

Ask your architect or mason.

It is mortar. Grout is for tile: ) , not stone exteriors. Mortar needs to pair harmoniously with YOUR stone. Which is what? On what home?

Some context attached to the question and no it is not grout but we need to know why you are asking.

I think its best to match the mortar color with the stone to allow the textrue of the stone to dominate instead of the joint pattern.

Thank you to all who responded. We are building a house and it is clad with limestone but the mortar that was applied is too dark and we are struggling to get the right color. We have tried to match the stone color but have not succeeded. Our goal is an old world look like in the photos.

Mortar takes many weeks to lighten up to it’s eventual color.

Frequently lighter color mortar for limestone is made with white Portland cement; it looks good with either the very light "Texas limestone" or slightly creamier or sandy colored limestone and doesn't have the gray or brown tones of some other mortars.