The mortar in my house seemed to have a high proportion of lime,and if I remember correctly I used about equal lime to sand for repairs. I think colour will depend very much on the nature of the sand you use - eg if it is clean or 'washed' - or whether it contains a lot of clay or such that would give colour. If anything, using a 'clean' sand, mine came out a bit too white.
There is a lot of more expert knowledge over on the Green Building forum. Use the search facility.

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try mix 1:1 silver sand and lime putty
 
then do a few trial patches round the side down low with various mixes

This thread has appeared in 'Latest Forum Posts' but without a post of todays date (other than this one) - the last being tony's dated 29 Sept. Why does that happen?

Oh yes! Been here! (This afternoon I was looking at a horrible grey repair somebody'd done on a Medieval church!)
Basically the dried colour of your mortar will depend mainly on the colour of your sand, although I fancy Lincolnshire NHL is whiter (and less eminently hydraulic)than Castle.
Varying the mix will just give you other issues.
I try to find a local (ish) sand for repairs and that often seems to do the trick. I guess that's because nowadays we cart the stuff about the country much more than earlier generations did.
3:1 is a good mix for a moderately hydraulic (NHL 3.5) pointing mix but your lower courses of stone will need a 2:1 mix of the same materials.
I find lime mortars do better with a wider particle size distribution than you normally get at the builders' merchant as 'builders sand' ... worth checking that for yourself.
Obviously, you won't be applying this outdoors in heavy rain or temperatures that will dip below 5 deg C at any point in the 24 hour period, though, will you?