Код:
<html>

<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1251">
<title>Радужный текст</title>
</head>

<body bgcolor="#000000">
<script>
function MakeArray(n){
   this.length=n;
   for(var i=1; i<=n; i++) this[i]=i-1;
   return this
}
hex=new MakeArray(16);
hex[11]="A"; hex[12]="B"; hex[13]="C"; hex[14]="D"; hex[15]="E"; hex[16]="F";
function ToHex(x){           
   var high=x/16;
   var s=high+"";               
   s=s.substring(0,2);          
   high=parseInt(s,10);      
   var left=hex[high+1];    
   var low=x-high*16;           
   s=low+"";                  
   s=s.substring(0,2);         
   low=parseInt(s,10);         
   var right=hex[low+1];       
   var string=left+""+right;    
   return string;
}
function rainbow(text){
   text=text.substring(3,text.length-4);     
   color_d1=255;               
   mul=color_d1/text.length;
var page
   for(i=0;i<text.length;i++){
      color_d1=255*Math.sin(i/(text.length/3)); // some other things you can try>> "=255-mul*i" to fade out, "=mul*i" to fade in, or try "255*Math.sin(i/(text.length/3))"
      color_h1=ToHex(color_d1);
      color_d2=mul*i;
      color_h2=ToHex(color_d2);
	page = '<FONT COLOR="#FF' + color_h1+color_h2+ '">'
	page += text.substring(i,i+1)+ '</FONT>';
	document.write(page)
   }
}
</script>


<p align="center"><font face="Fixedsys"><span style="background-color: rgb(0,0,0)"><script
LANGUAGE="JavaScript">
rainbow("-->Радужный текст<!--")
 //-->
</script></span></font></p>
</body>
</html>