Thenk you guys. I solved the problem in this way:

<script type="text/javascript">

var s='First JavaScript string.';
var c=' This is second text.';
var mString=s.concat(c);
var x;
for(var i=0;i<mString.length;i++) {
if(i % 2 == 0) {
x = mString[i];
x = x.fontcolor("red");
document.write(x)
}
else {
x = mString[i];
x = x.fontcolor("black");
document.write(x)
}
}

</script>