I've been told by the bricklayers on my site that the brickwork should be judged around two weeks after it was done, and after the bricks have been washed. That way you can see the real mortar colour and are not blinded by dirt. We'll see, but so far I have no reason to think it won't turn out ok.
You are correct on both counts, we judged the finish product. And yes, it was probably the mixture of the mortar that has caused the problem. Very rare it happens, but does happen.