
Honestly - think your contractors ought to lift and re-lay the replacement flags for half price.
They have been (or ought to have been) in the trade long enough to a/identify weathered from different batch and b/different texture slabs - they were aware that they looked different so this should have been in the back of their minds already.
They continued to lay, presumably hoping that either you wouldnt care or the others would weather to a similar colour quickly.
Therefore, I do think they are also at fault - IF they asked you and were upfront about the possibility that these flags would not end up the same colour as the others and were from a different batch and then you told them to go ahead anyway, then it is your fault.
Dont really see how it is the suppliers fault if either the contractors carried on, or you told them to carry on once you were aware it was a different batch.