I mean, re-reading is meant to be used once you've already finished the series once right? Besides, setting a manga to "Finished" then back to "Reading" resets the read status of every chapters, so re-read is... kind of pointless tbh? I honestly doubt it'd be worth the minimal effort to change the colour of the "re-read" option since the function itself feels kind of redundant.
Edit: Thought thinking about it, I guess an argument can be made for organising follows with multiple pages of Manga, but for "re-read" to serve that purpose, the colour of the option is a none-issue due to filtering out everything else, so my opinion on changing the colour is the same, but maybe it isn't quite as redundant as I first thought.