On Earth, while birds and reptiles can be quite brightly colored, it's rare for mammals to have any bright colors, and the rare times they do, it's either just slightly different (like with orangutans) or from outside sources (like symbiotic algae). There are several interlocking reasons for this - most mammals don't have good enough color vision for it to have real evolutionary value, there isn't enough bright color in most environments mammals live in, and melanin doesn't have brightly colored variations.
I'd like an Earth-like planet, with overall near-identical flora and fauna, but where there's one mostly isolated ecosystem that has brightly colored mammals. How could I make an evolutionary pathway that leads to that happening?
(I would preferably like for the environment with the brightly-colored mammals to be a temperate rainforest for unrelated worldbuilding reasons, but I'm fine with a different environment.)
Thanks in advance.