El idea fue de Julián.
Male would be el
Female would be la
You are correct when saying that el is for masculine nouns and la for feminine nouns, so in this case it'd be "la idea" because idea is a feminine noun. You have to match the article with the noun that comes right after, not nouns later in the sentence.
I hope this clarified it.
HI again Sam
just to clarify a bit further, Julián is indeed a masculine proper noun, but the article needs to match with the noun right after it, which in this case is "idea", not "Julián", so:
La idea fue de Julián.
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