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Contrary to what the others are saying, Magpul's new polymers don't compromise integrity for colors. In fact, I believe that was only an issue on the original Gen 1. At least that's what Magpul said.
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Without getting into much detail, colors will always affect the part during molding.
Sometimes the difference is too small to detect but other high stress items the difference is measurable.
Black is always the strongest (outside of neutral -nocolor) due to the carbon molecule size.
Dying afterwards and painting does not affect the strength of the part after it is molded.