There are loads of colourful fish you could add to a tank that size. Let us know what kinds of fish you are interested in so the choice can be narrowed down.
If you like livebearers, guppies and platties come in a variety of colors and are good to get started with.
Cory cats, while not the brightest colors, come in many patterns and will add interest and activity to the bottom of your tank. They are schooling fish, but you could keep 3 happily
Glowlight tetras and neon tetras are both small, colorful fish, but would need to be kept in small schools of 7 to 9 at minimum.
All of the above fish are peaceful and good community tank citizens.
if youre looking for easy care fish, go with platies or fancy guppies, as inchworm suggested. huge variety of colors. depending on what elseyou put in there, a dwarf gourami of some type might be nice. bloodfin tetras or harlequin rasboras can give you a splash of red and you have room for a nice school of each. and again, like inchworm said, corys are great little bottom dwellers that won't add much color but will definitely add character to your tank.