Nothing looks worse than a repair to pointing, than a mortar mix of the incorrect colour, some of the pre-mixed mortar mixes have very high cement content or made with dark sand, and both will stand out like a sore thumb if used to repair a wall.
The best way is to mix up a small amount before you start the main job, measure the mixture using a small cup etc (use a bucket for normal mix). A starting point would be 1 part Portland cement to 4 parts Sand. The best idea would be to mix up several mixtures one at 1 to 3, 1 to 4, 1 to 5 and 1 to 6 let them dry and see which is the closest to your existing mortar.
There are many types of sand and each type will produce a different coloured mortar when mixed with cement, if your mortar is very light or dark colour take a sample of the loose mortar to the local builders merchant, he may have supplied the sand or be able to help you locate some. Some mortars are artificially coloured with a cement dye making it even harder to match.
If you are starting a project you can decide what mix to use see table below.
The lime is added to slow down the drying process and to help the mixture flow, however many builders use a plasticiser instead of the lime this also improves workability of mortar and minimises cracking and crazing, follow manufactures instructions on the amount to use.
Mix together with water until you get butter like consistency, slipping easily off the trowel but still holding its shape if you make a depression. Don’t use the mix after 2 hours or less in very hot weather.