If you are looking for color - go for rainbowfish!
With that size tank, a school of 7 or so melanotaenia boesmani with their bright purple, blues, yellows and oranges or m. lacustris with their amazing marine-like blues would look amazing. They get to 3-4". Alternatively, the stunning glossolepis incisus gets to about 5" and is an amazing red color.
Don't let their drab colors in the LFS fool you! Rainbowfish color up amazingly once settled in and they are by far the most colorful schooling fish. Plus they are active in all levels and, if in a large enough group, are very out-going and entertaining fish.
Then I would suggest a colorful loach as a bottom dweller. Maybe get something like the gold zebra loach (botia histrionica) which only gets to 2.5" so you can put quite a few of those in. Alternatively, the larger yo-yo loach ahs attractive, contrasting markings and is quite active as well.
With the rainbows, you don't realy need any top dwelling fish but if you still want some I'd go for a trio of moonlight gouramies which get to around 6" (1 male, 2 females). You could also try opaline, gold, cosby, platinum, blue or (collectively) three-spot gouramies but these are more aggressive though slightly smaller, maxing out around 5". Thick-lipped (colisa labiosa) or banded (c. fasciata) gouramies are also possibilities and deffinately have stunning color and are quite peaceful. They get to between 3" and 4". The last option would be pearl gouramies which would also look amazing and are peaceful fish. They get to be between 4" and 5" but can be quite shy if bullied by their tankmates. They should be fine with most rainbows and wouldn't mind the loaches.
The purple spotted gudgeon is another smallish, very colorful species you may want to consider. It should get on with a big school of rainbows.
Obviously other great fish for a tank like this are the apistos, rams and other dwarf cichlids. I personaly would suggest kribs or olivian rams if that's the sort of route you decide to take. One of my favourite cichlids, though not that colorful, is the keyhole. They are shockingly shy and peaceful for a cichlid and breeding them is quite easy too.
If you want a cichlid tank with color though, what better choice is there besides discus?
Or, if you aren't up to all the maintainance and want something easy, maybe get a few angels, some scissortail rasboras, a group of some kind of cory and a bristlenose plec.
Something else to consider is tiger barbs... There are three main types and a mix of these can be very colorful. A big school of barbs - of any color - is an amazing sight and will get along with most other fish as well (they aren't as nippy in a big group). I'd probably put them in with some medium-sized loaches and kribs and maybe an RTBS or rainbow shark as well.