I understand the product to be a "foam". Could it be the surface condition making it appear dull? Is it extruded or molded? either way, perhaps looking at mold surface condition or mandrel condition could help.

I also saw a link that stated that scorching may be an issue. Are heats in the right ranges?

I used to work for a plastic packaging company, and found that the night shift would load the black colorant to be sure they did not drift light out of spec. This allowed them to ignore the black lines. They would also speed the process to make efficiency look good. The result was overloaded color in the parts, that we cooked due to the added heat due to shear in the barrell. The high temp and color concentratin killed the appearance of the parts, and also made them more brittle as the polymer chains in my PP were broken down.

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