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> Feathered dinosaurs are known to be colorful

Perhaps because it's not truly known.

> The discovery "suggests a more colourful Jurassic World than we previously imagined,"

-- from your link

Bacteria can be colorful. It can also fluoresce.

My blood is red because it has iron in it not because there's an evolutionary link to my eyesight.

You're very likely right there's no causation in that direction, but it seems entirely possible that we experience red as a vivid color in part because noticing blood is evolutionarily important.

The article likely distinguishes between simply having colors versus evolving colors specifically for sexual selection - fossil evidence can show dinosaur coloration but determining behavioral purpose is much harder than observing living fish species.