Is the colour of a chrysalis predetermined?
Generally, in biology nothing is designed but if something is working it is saved for the future generations. Chemistry of colors is a complicated field, you can't just slightly modify single "color" molecule to get any color you want. Maybe in chemical sense the material of this shell can be easily modified to get shiny. Suppose it happened occasionally once due to sporadic mutation and no one dared to eat this chrysalis, which gave abundant progeny.