Jag löste det.
<script type="text/javascript">
function addera(theForm){
var summan = 0;
var color = "black";
for(var i=0; i<18; i++){
summan += (isNaN(parseFloat(theForm.elements[i].value))) ? 0 : parseFloat(theForm.elements[i].value);
}
if (summan<30) color="blue";
else if (summan<36) color="green";
else if (summan<40) color="red";
theForm.summa.style.color=color;
theForm.summa.value = summan;
}
</script>
<input type="text" value="0" style="text-align: center; font-weight: bold; color: blue;" size="1" name="summa">
/S