Colorful, easy to care for and small.. well that includes dozens of colubrid species (kings, milks, corns, rat snakes, etc etc) Most of them don't need anything bigger than a 20 gallon tank when they're full grown. I'll show you a few of mine:

albino nelson's milksnake


normal and anerythristic kenyan sand boas (not colubrids, but they fit in your requirements)


greybanded kingsnake (the rest of him continues this pattern)


tangerine honduran milksnake


if you want an all black snake.. mexican black king


hypomelanistic tangerine honduran milksnake


the one hanging out on the cage is a jungle carpet python.. the one IN the cage doesn't fit your requirements.


normal and albino stripe california kingsnakes


Whew that's a lot of pics.. if you have any questions, don't hesitate.