I'd have to say tropical birds.
Not necessarily. As trichromats, human color vision is better than most, and we're not particularly "colorful".
peacock
All of them are colourful but after seeing pictures i would say the Mandarin fish.
Camelion
Just like most male animals - it's for attracting a mate. The females are attracted to colour, so the males evolved to have colourful plumage.
maratus volans(peacock spider)
more colorful, most colorful
It's either a peacock or a parrot you decide
To attract animal/insects to them.
Most Koi carp are colourful
Depending where certain types of bugs live, most colourful bugs live in rain and tropical forests'. They may have bright colours and they might be very colourful because they show that they are poisonous and to keep bigger animals away so they do not eat them, even tough they may look small and weak, their colours give them a sign to mark that they are poisonous.
The petal is the colourful part of a flower that attracts pollinators. It is often the most visually striking part of the flower and plays a key role in the reproductive process.