I want to mix white cement and silver sand to get the right colour jointing to replace existing mortar. I can't find silver sand locally, can I use an alternative sand?
I have no idea what silver sand is.
I would be tempted to pry out a chunk of mortar from the existing mortar and go to a specialty tile shop. "Hi, can you match this?"
That said, matching mortar colours is a crap shoot. You may find that to get a match, you will need to scrape out 1/8" of the existing mortar everywhere, and redo it.