Streaks left on siding after power washing?

After borrowing a power washer, we tested it on the backside of the house with no problems. It worked great and did a nice job of getting the dirt off the siding. When we did the same to the front of the house, we noticed later in the day that it looks as if someone did graffiti all over! There are streaks all over that are darker than the the rest. It's very noticeable and we feel just sick that the siding may be ruined. Any help on what our options are to fix this?