What did you try and wash, the bright or the dark as Marty asked ? I see this every year as the dirt builds up over the winter due to no rain. I pressure wash and it brings it back to match the clean area that was under the cover. You will need a lot of pressure and have the wand set to only an 1" wide spray and be very close to the surface, slow going and will take many hours. I am experimenting with a sealer this year to see if the "clean up" time can be reduced.
Be careful with a pressure washer on concrete. The jet tips can etch the concrete. A 40 degree tip is suggested for cleaning. I'll be cleaning mine this weekend with a new cleaner. I'll let you know if that helps.