Using ready-made mortar problem.

Mods - please move if appropriate. I used a ready-made mortar mix to replace the old mortar between the paving flags on my patio but it dries to a very pale grey colour like plaster instead of a dark brownish colour like the old stuff. It looks awful but fortunately I've only done a little bit. Having made a mortar mix using plaster and sharp sand before when I did the same job at our old house which dried fine, I checked the bag description as the contents look and feel exactly like plaster but it does say ready-mixed for mortaring. Should I add some sharp sand to get the brown colour and, if so, how much?