So, I take it you know
how to color your nickname since you said you have one... but in a single color?:
example:
/nick [c=4]my nicname[/c]
Now, to create a gradient as shown in your screenshot, you need to set a starting color and an end color, like so:
/nick [c=4]my nickname[
/c=10]
In the above exmaple the nickname starts with color 4 and will end with color 10, thus creating a gradient of colors from color 4 to color 10...
(PS: try to use a PNG file format next time for your screenshots, the quality will be much better 
)
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