Can Flagstone Lose It's Color?

We spent a few thousand dollars on flagstone for a patio slab and are sick to our stomachs as the stone has apparently lost it's color!! It turned a cement like color and several of the stones are gray color. Has this ever happened to anyone?? Is it defective stone??? The company says it has to be something the installer (a separate party) did and it is not the stone. HELP APPRECIATED!!