Question

Why was spinarak purple in gen 2 and then green going forward?

Comments

the early sugomori artwork depicts it as green like normal, in case anyone was wondering.
a post on this gamefaqs thread suggests that it's because it can only be found at night, so it's darker because of the implied shadow. this would also make sense because gen 2 introduced day/night cycle and they were probably still figuring out how they wanted to use the GBC's color capabilities.

https://gamefaqs.gamespot.com/boards/960099-pokemon-heartgold-version/50735384

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Hint: it's only purple in G/S