Mbuna. They're African cichlids found in the freshwater Lake Malawi. Examples include Labidochromis caeruleus, Pseudotropheus demasoni, Cynotilapia afra, and many, many more. These cichlids are often very brightly coloured, especially with vibrant yellows, blues, and whites, which are otherwise not seen all too often in freshwater aquariums. Their colouration makes it hard to believe that they aren't saltwater fish at all! The only downside is that they are known for their aggression and territorial behaviour.
Of course, there are fish like pufferfish and pipefish that look very similar to their saltwater counterparts, but assuming OP is looking for freshwater fish that rival the flawlessly beautiful colouration of commonly-kept saltwater fish, I think Mbuna are the best candidate.