Ask if you can view a sample board. Even better, our GC purchased one package of each color and laid them on the roof so we could compare prior to making a decision.
From the photos I still have of the sample board, the colors used in Oyster and Pewter appear identical but the proportions are different (i.e., Oyster has more of the lighter, blue-gray pieces whereas Pewter has more of the darker, browner pieces, and both have even less of the same very dark pieces).
We ended up choosing a different color, so I can’t tell you what an entire roof is like. Sorry.