<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

    <targets>
        <target name="richTextBox" xsi:type="RichTextBox" controlName="richTextBox1" formName="form1" useDefaultRowColoringRules="true">
            <row-coloring condition="contains(message,'serious')" fontColor="Red" backgroundColor="Blue" style="Underline,Italic" />
        </target>
    </targets>

    <rules>
        <logger name="*" minlevel="Debug" writeTo="richTextBox" />
    </rules>
</nlog>