How can I edit the appearance of a citation?

You’re on the right track. Using a show rule is the way to go here.

#show cite: it => {
    show regex("\d{4}"): set text(blue)
    it
}

In case you’re unfamiliar, you can read about regex, or “regular expressions”, here. regex("\d{4}") means “match four-digit numbers”.

Note that there will still be a hyperlink for the whole citation, not just the year.

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