Help With Coloured Usernames

When i have set a specific rank..say you need 15 posts to be able to view certain threads.
And when they get the 15 posts how can i make it so the colour of there name changes automatically to Blue?

Code:

<font color="blue">WORDS HERE!</font>

there you go

where do i put that code?

In the text area of the rank you create

thanks for the help i know what to put it now