Devo fare in modo che quando arriva a dieci caratteri rimanenti, si colora di rosso

Codice:
<script type="text/javascript">  
charCount = 1; maxCharCount = 2000;  
function displayRemLength(fieldName) {  
remField = document.getElementById(fieldName);  
remField.innerHTML = (maxCharCount - charCount > 0) ?  
maxCharCount - charCount : 0;  
}  
function evalEntryLength(curField, maxLimit, discardXtra, errClass, normalClass { 
maxCharCount = maxLimit;  
var fieldLength = getCharCount(curField);  
if (fieldLength > maxLimit) { if (errClass != "") {  
curField.className = errClass;  
} 
if (discardXtra) {  
showAllowedLength(curField, maxLimit);  
}  
}  
else if (normalClass != "") {  
curField.className = normalClass;  
}  
}  
function getCharCount(curField) {  
charCount = curField.value.length;  
return charCount;  
}  
function showAllowedLength(curField, maxLimit) {  
curField.value = curField.value.substr(0, maxLimit);  
window.status = curField.value;  
}  
</script> 

<span id="charCount">100</span> caratteri rimanenti 

<textarea name="messaggio" id="text-guest" cols="0" rows="0" onkeyup="evalEntryLength(this, 100, true, '', ''); displayRemLength('charCount');></textarea>