6" gives you a huge variety in your choice of turtles (and the relative price). Maybe describe your preferences in more detail as to brightness of coloration, exotic appearance, absolute hardiness, personality etc. What comes to mind immediately are captive-bred (and ONLY captive-bred) Spotted turtles (Clemmys guttata) that are colorful but stay small. They are always in the classifieds, so you can check out prices (usually $100 plus). Get the largest juveniles you can afford, however.