In your event page it is calling a template. That template is eq2ui_event_template.xml
There is also a compact version.
I didn't do to much snooping around, but if you open up the template file, change the color in there, then go back to the guild window the color change should stick. I would rename the template file to like, my_template, have your guild window reference it instead of the default one.
This should lead you down the right path, best of luck.
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