> As a result of events that occurred during the early history of mammals, eutherian mammals retain only two of the four cone opsin gene families found in many other vertebrates. Very likely during this same time frame, the elaborate system of coloured oil droplets characteristic of photoreceptors in many vertebrates were also abandoned, as was a portion of the specific retinal circuitry dedicated to processing colour information (Jacobs & Rowe 2004). These changes left most eutherian mammals with a single dimension of colour vision. Primates subsequently escaped this restriction by evolving a series of visual system alterations that provided opportunities for expanded colour vision.

From Evolution of colour vision in mammals

This means that mammals generally have poor color vision, primates like us being an exception. No need for flashy displays, they couldn't see them.

As for birds (aka avian dinosaurs), a lot of them have four color receptors, one more than our three, and their plumage is in fact more complex and dazzling than what we humans can even see. (Similar to flowers who sport designs only visible in the ultraviolet, meant for insect eyes.)

Fishes generally have good color vision, even into the ultraviolet, but as light decreases with depth those living deep down can be left only with rods in their retinas, i. e. be color blind.

In both cases, fishes and birds, the display of colors is used to attract mates, sometimes to scare rivals away.

Fantastic shapes (which can be found in mammals too, in the form of weird, cumbersome horns, for example) are thought to be the result of sexual selection, the females (generally) "preferring" these as a sign of health and thus good genes and lack of parasites. (A bird with a ridiculously oversized tail has to be good at survival to make it to the mating season.)

A last point about fishes: water drag increases with temperature. So while the big ones like sharks, tuna, &c., are always streamlined, it makes no sense for small tropical ones, and they can indulge in attractive body forms instead. But you don't see that in cold waters.