Housebuilding Sand

There are many many types of sand and each type will produce a different colour mortar when mixed with cement. Your property may have been built from a local sand or it may have been imported by a developer who got a great deal from a quarry further afield.
If your house is relatively new it may be worth talking to the local builders merchants to see if the development was built using materials ordered through them. You can try to contact the developers or even the actual builders. Life will certainly be easier if you can track down the sand used.