Sherwin Williams Deck exterior super deck stain

Hello. We had a new deck built 2 years ago. Used pressure treated wood and let it sit till last year before staining, as recommended. We used a Sherwin Williams exterior Super Deck Stain, one of the colored ones. (It is a light purple). The painter recommended using Thompson’s water seal on it, and though it does a good job repelling water, as evidenced by water puddling on it, it has turned a rather dirty color over the winter. I scrubbed on hands and knees to see if it was dirt, but appears to be a permanent discoloration. Do you have any ideas as to what went wrong, or what we should have done?