I can never remember what the different Visualforce page message options look like between the severity and strength so here are the options along with the code.

<apex:page title="Visualforce pageMessage Samples"> 
<apex:form > 
<apex:pageMessage title="Message Title" summary="Strength:0, Severity:Confirm" strength="0" severity="confirm" /> 
<apex:pageMessage title="Message Title" summary="Strength:1, Severity:Confirm" strength="1" severity="confirm" /> 
<apex:pageMessage title="Message Title" summary="Strength:2, Severity:Confirm" strength="2" severity="confirm" /> 
<apex:pageMessage title="Message Title" summary="Strength:3, Severity:Confirm" strength="3" severity="confirm" /> 
<apex:pageMessage title="Message Title" summary="Strength:3, Severity:Info" strength="3" severity="info" /> 
<apex:pageMessage title="Message Title" summary="Strength:3, Severity:Warning" strength="3" severity="warning" /> 
<apex:pageMessage title="Message Title" summary="Strength:3, Severity:Error" strength="3" severity="error" /> 
</apex:form> 
</apex:page>