Granite

Need your help,  Just had granite tops installed in kitchen, there is an L shape that required a seam at the seam it's about a 3 foot piece attached to the L, the problem is the 2 pieces are one lighter one darker, they are the same colors just one lighter, the installer said he has been doing this for 11 yr's and he promises that in time the water will dry up and the 2 will match. he said he thinks that when they cut it in the shop they sealed one section and not the other. and the sealed section is dark cause it was a little wet, so he came and sealed the 3 ft section it's still one side darker than the other. he said it's common and that eventually they will dry and match, so is there anyone out there who agrees and give it time or is this not right and should they replace all o0f it, My customer will do what I say so i need someones advice who knows thier granite  John L.