Here is how you would create that extension (and don't forget to activate it before loading a campaign like I always do)
1) Create a new directory in your "extensions" directory named in a sensible way
2) In that new directory, create a file named "extension.xml" with the following contents (or similar):
<?xml version="1.0" encoding="iso-8859-1"?>
<root version="3.0" release="4">
<properties>
<name>PFRPG2 Combat Tracker Text Color</name>
<version>1.0</version>
<author></author>
<description>Chnage Combat Tracker Text Label Color</description>
<ruleset>PFRPG2</ruleset>
</properties>
<base>
<template name="label_ct">
<label>
<color>000000</color>
</label>
</template>
</base>
</root>
Note: that is going to change the color on any control using the "label_ct" template, not necessarily just the header line. (Of course that seems to be specific to the Combat Tracker, so it should only be within the Combat Tracker.)