The myth of "cheap Chinese plastic" really needs to go away. It's not like ABS is some mythical formula only found in Denmark—it's a common material for manufacturing the world over. The only reason Lego's use of it is even remotely notable is that their competitors mostly use cheaper plastics.
The more likely culprit for fluorescent reds and greens is the dye, which Lego sources from various distributors around the world. In China, the dye, like the plastic, is sourced locally, not because Chinese dye is phenomenally cheaper, but because if you're manufacturing in China anyway (which Lego does for specialty parts like figs that benefit from cheaper LABOR, not cheaper MATERIALS), it's vastly impractical to ship either plastics or dye across the world and pay ridiculous import fees for something you can get for market price within the same country. The fluorescence is an inconsistency, but not because of any difference in cost so much as an aspect of color-matching that Lego doesn't test for. Why should they? Lego has literally never included a blacklight in a set or designed a set with that specific display method in mind
I'm really sick of the widespread and slightly racist notion that all Chinese goods are sub-par quality. Plenty of high-quality manufactured items, from iPhones to Lego, are produced there, simply because laborers there are accustomed to much lower wages than workers elsewhere—a boon for complex or detailed work that can't be automated to the same extent as other goods. There's an argument to be made that outsourcing labor to countries like China is exploitative and only perpetuates the lower standard for human rights there, but in all the whining I've seen from AFOLs about Chinese production, I don't think I've EVER seen anyone whose primary concern was the standard of living for the workers who made their plastic toys. Nope. All they ever care about is the end product, and if there are any deficiencies at all (real or imagined), the squinty-eyed Asians are the easiest scapegoats.