For color and shape...a yellow watchman can give you brilliant yellow. A couple of yellowtail damsels can give you yellow and brilliant blue---so could a blue star damsel if you have LED lighting---they're not so brilliant under regular. The shape---a flame angel (one of the small species angels) could give you red, if you don't have corals---angels cannot be trusted with stony coral, but a good softie tank can probably outgrow any damage they do. There are fairy wrasses, which come in reds and oranges, and there are the red firefish (screen your tank against jumpers with those: they're amazing at it, also in going through drain slits in a downflow box and ending up in the sump.) Look at some of our sponsors' fish and invert catalogs for some ideas, and look at the adult size on the critters---max 5" is possible in a 55. And the better you can get your water quality and lighting, the more easily you can keep corals, some of which will grow like mad.
The deal is, once you limit your size to 3" to 5" adult, AND consider varieties of coral, the 'bigger' your tank gets in terms of space: ---it sounds like a paradox, but corals 'fold' space, and create tunnels and alleys and shapes that make the space complex, and give fish that live in the coral maze much more to do than to go up and down the glass.
If that turns out to be a route you like, we can tell you how to encourage coral growth: it's mostly having a sump, having a skimmer, having decent lights, and being willing to run water tests once weekly. The good news is that corals are living filters, and actually help the tank rather than piling up bioload.
SO whatever you'd like to do within the realm of 3" to 5" we can help you figure options and find a solution. I ran a very successful 52 gallon lps reef that grew like mad and was covered in corals---mostly hammer. I've gone to a larger tank now, a 102, in which I keep mostly damsels, gobies, and a basslet---again, quite colorful fishes, and in which I'm working on growing corals.