Change the color in apex:panelBar.

Code Format:

<apex:page sidebar="True">
    <apex:panelBar style="background-color:Orange;" headerStyle="background-color:lightgreen;background-image:none;">
        <apex:panelBarItem label="Line 1">
            Line 1
        </apex:panelBarItem>
        <apex:panelBarItem label="Line 2">
            Line 2
        </apex:panelBarItem>
        <apex:panelBarItem label="Line 3">
            Line 3
        </apex:panelBarItem>
    </apex:panelBar>

</apex:page>

Output: