Please use the extension manager to install the plugin, no configuration needed.
Place this code sample in your playground:
<color green>text</color> <color blue/lightgrey>text</color> <color #FF0000>text</color> <color /#FFff00>text</color>
and you will see:
This plugin supports the X11 color names
This source for this plugin is shown in the Sample Plugin Tutorial.
Christopher Smith seems leaved the plugin unmaintained. Another user leeyc0 provided hosting of the plugin in the github, but have no time to maintain it. Anyone interested in maintaining may leave a ticket in github and leeyc0 will be glad to hand over the project.