This can be a real problem to match old coloured mortar with new.
You could try using a very light coloured sand with the dye as very orange sand can overpower the dye.
However, for best results you can't beat readymix mortar, see this pdf link
http://www.tarmac.co.uk/live/pdf/product/mortar_colour.pdfThey do many colours including Chololate Brown and Black Brown, and each of these in various shades. The advantage of this is that the manufacturers know what will be the finished colour
after curing. You can't get this with these diy dyes you just chuck in the mixer.
Not sure on cost of this stuff though.............