The birds are generally more colorful than they appear once you have them living on your island and can see them from every angle.
Dirty Jacob stands out here.
Can't go wrong with Pietro. Pango, Phoebe, Julia, and Judy also have a fair number of colors. But most villagers have 1-2 bright complementary colors, so you could also try using 7 villager spots to have each one a single color from the rainbow. Like Octavian for red, Tangy for orange, Eloise for yellow, Sprocket for green, Sherb for blue, Sydney for indigo. Honestly I have trouble telling the difference between indigo and violet. I guess Violet the gorilla would be the right shade of purple for violet, right?
Of course Pietro and of course he's been mentioned so many times already
This might be good time to mention how much I prefer sheep design in the previous games. The scarves were so unique and Pietro looked even more colorful. Just look at him!