I have purchased "Dekorsten Väggsten Used Brick Milk" but now realize that I don't know how to solve the grouting, i.e., to get the right color and appearance.
In the installation instructions, it says "For grouting, use, for example, mortar and plaster of class B. Consumption 6 - 8 kg/m2 depending on the width of the joint" and it should be sprayed out and can advantageously be reworked with a brush to get an old & used & rustic feel.
As I understand it, Used brick milk is towards the cream-colored side, and it would probably look strange with a gray or white joint. Looking at the advertisement image, the joint seems to be the same/similar color as the wall stones. It blends in, at least.
Cement gray (slightly warm gray) grout looks good aesthetically, but if you want the exact same tone, you should get mineral pigments to mix in. Light umbra might work. Bruksspecialisten has this.