I was curious how this works and I think this might be a bug in the “view” when replying [1]. Because if you send that email, and load it in the “regular” mail view, the colors are correct 
I think they might be stripping too much style in the reply view, or they don’t do the same “processing” to handle light vs dark mode.
or we haven’t found the proper incentation that Apple is expecting to correctly handle dark mode. ↩︎