You need to get the plugin essentials:
http://forums.bukkit.org/threads/gen-sec-econ-tp-admn-essentials-v2-7-2-a-collection-of-useful-commands-1597-1818.15312/

Then once installed and configured, in game type:
/nick [user] [name]

For "[name]" you can also add color codes:
Here is a list...
http://www.minecraftwiki.net/wiki/Class ... olor_Codes
Example: "/nick AwesomeMarioFan &1AwesomeMarioFan"
(&1 will color the text after that with that color until you put another one in to change the color.)

If you want a plugin that includes ranks along with colored names or need more help, please feel free to message me I have been running a server for over 2 months.