Mortar is easy to do--it's just really slow if you don't know the correct way to do it. Still, you can do a lot in an afternoon.
With bricks mortar tends to leave hazy residue behind when it gets where it wasn't supposed to go, even if you diligently wash it off. You have to come back with acid to remove it. That works fine with bricks, but with rocks... maybe you need to be more careful. Certainly I wouldn't do it with, say, dyed concrete pavers, since the acid takes the finish off them. You could probably still do it, but you'd have to be really neat with the mortar and I just wasn't.
But as far as matching the mortar, that turned out to be easy. You can dye the mortar or you can weather it afterwards pretty easily.