Actually, there are many colorful natives that don't need a large tank.
Here's three that can be kept in a 10 gallon:
The golden topminnow (FUNDULUS CHRYSOTUS)
http://tinyurl.com/d4r5j
The flagfish (Jordanella floridae)
http://tinyurl.com/9lxsa
The bluefin killifish (Lucania goodei)
http://tinyurl.com/7h534
There are many, many more.
Jon