I understand your opinion, but I think no one in the fandom considers using the original words of Jane Austen as plagiarism. I have seen many authors indicate as an Author's note that they have used JA's own words because no one could find better if a canon situation was to be described; also, it's a wonderful source of humor to find some better known sentences placed in a different context: e. g putting the words of the unforgettable dialogue between Lady Catherine and Elizabeth into some other character's mouth, or putting a different spin to the Hunsford proposal dialogue. I think it is considered enough to give proper credit.
I find it jarring if the different font colors draw attention to the quotes, especially if (very correctly) the pronouns modified to fit the situation stand out as non-quote color. (It would be especially difficult to follow three different font types (if, as I hope, you refer to the N&S crossover you mentioned before)
How about making an author's note at each chapter to indicate where one could find the source material, without highlighting it directly in the text with different fonts? E. g. at the end of the chapter you might list, Pride&Prejudice, Ch. ---, Presuasiojn, Ch. ---, maybe if you want to be very exact, you could add an upper index number to the part you quoted in the text (example
1, example
2). This way, anyone who wishes to look up the exact source material will be able to do so.
1Persuasion, Ch. 1,
2Pride and Prejudice, Ch. 39