Do momiji and kouyou mean the same thing?
According to Jisho, yes. They both seem to be readings of 紅葉. I could be unaware of nuances, but maybe another more advanced user could help you with that.
In general I’d recommend posting your question with the relevant kanji, as the romaji (or hiragana) might not be unambiguous.
もみじ in a narrow sense is a specific tree, the Japanese maple. But in a general sense the word can be used to refer to all color-changing foliage in the fall.
こうよう means color-changing foliage.
So in some cases they can be interchangeable. In other cases, no.
And as mentioned, they can both be written as 紅葉
I spoke too soon, thanks for additional information!
As an offtopic question, is there any chance of a new entry in the ‘things wanikani doesn’t teach you’ series in the (near) future?
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