You can try dyeing the new mortar mix to match but this is nearly impossible to do with any accuracy. The mix looks darker when wet than dry so how can you judge what the finished tint will be like? If you still want to try this you can get a tub of "Cementone" or similiar from a builders' merchant for around €14, it comes in various shades and is added to the dry mix before adding water. If the stone wall is very old it would probably have been built with a lime mortar as cement is a fairly new material. Try adding some lime to the mortar to give a little bit of plasticity to the mix and avoid making a very strong mix. When completed soften the new mortar joints with the head of an old broom to roughen the joints and when fully set (leave for a week or so) you can try and weather in the new work to match the old. The best way to do this (and I kid you not!) is with a watered down solution of cow's manure or some other organic matter- this will promote the growth of lichens and algae etc on the new work and over a few months will give you that weathered look you require.