Trying to match your current mortar colour will be a trial and error process. There is pre-mixed coloured mortar available online but it is difficult to tell the actual colour from the screen.
Because your current mortar has been weathered any pre-coloured mortar will not match what you have. The traditional way of colour matching involves using a bag of Dingo 10kg Mortar Mix and a bag of coloured Oxide. By adding some of the Oxide to the mortar and comparing it to the current colour of the mortar in the wall, you can slowly build up to the colour that is close to what you need.