How can I get rid of weather marks on Trex decking?
The original Trex product fades over time and with sun exposure since it's partially composed of wood fibers. Conversely, it may darken where fungus is allowed to grow, such as where it contacts other lumber. 

I'd expect the weather marks to lessen in a few months or a year. I'm not sure it's reasonable to expect to remove them, though some of the contrast may be due to soil and fungus. Try a pressure washer with some detergent and then give it some time to even out. 

If that fails to provide a good outcome, a penetrating stain can be applied to create a more uniform appearance.