SEU TUTORIAL

<script type='text/javascript'>

//<![CDATA[

var rate = 20;

if (document.getElementById)
window.onerror=new Function("return true")

var objActive; // The object which event occured in
var act = 0; // Flag during the action
var elmH = 0; // Hue
var elmS = 128; // Saturation
var elmV = 255; // Value
var clrOrg; // A color before the change
var TimerID; // Timer ID

if (document.all) {
document.onmouseover = doRainbowAnchor;
document.onmouseout = stopRainbowAnchor;
}
else if (document.getElementById) {
document.captureEvents(Event.MOUSEOVER | Event.MOUSEOUT);
document.onmouseover = Mozilla_doRainbowAnchor;
document.onmouseout = Mozilla_stopRainbowAnchor;
}

function doRainbow(obj)
{
if (act == 0) {
act = 1;
if (obj)
objActive = obj;
else
objActive = event.srcElement;
clrOrg = objActive.style.color;
TimerID = setInterval("ChangeColor()",100);
}
}


function stopRainbow()
{
if (act) {
objActive.style.color = clrOrg;
clearInterval(TimerID);
act = 0;
}
}


function doRainbowAnchor()
{
if (act == 0) {
var obj = event.srcElement;
while (obj.tagName != 'A' && obj.tagName != 'BODY') {
obj = obj.parentElement;
if (obj.tagName == 'A' || obj.tagName == 'BODY')
break;
}

if (obj.tagName == 'A' && obj.href != '') {
objActive = obj;
act = 1;
clrOrg = objActive.style.color;
TimerID = setInterval("ChangeColor()",100);
}
}
}


function stopRainbowAnchor()
{
if (act) {
if (objActive.tagName == 'A') {
objActive.style.color = clrOrg;
clearInterval(TimerID);
act = 0;
}
}
}


function Mozilla_doRainbowAnchor(e)
{
if (act == 0) {
obj = e.target;
while (obj.nodeName != 'A' && obj.nodeName != 'BODY') {
obj = obj.parentNode;
if (obj.nodeName == 'A' || obj.nodeName == 'BODY')
break;
}

if (obj.nodeName == 'A' && obj.href != '') {
objActive = obj;
act = 1;
clrOrg = obj.style.color;
TimerID = setInterval("ChangeColor()",100);
}
}
}


function Mozilla_stopRainbowAnchor(e)
{
if (act) {
if (objActive.nodeName == 'A') {
objActive.style.color = clrOrg;
clearInterval(TimerID);
act = 0;
}
}
}


function ChangeColor()
{
objActive.style.color = makeColor();
}


function makeColor()
{
// Don't you think Color Gamut to look like Rainbow?

// HSVtoRGB
if (elmS == 0) {
elmR = elmV; elmG = elmV; elmB = elmV;
}
else {
t1 = elmV;
t2 = (255 - elmS) * elmV / 255;
t3 = elmH % 60;
t3 = (t1 - t2) * t3 / 60;

if (elmH < 60) {
elmR = t1; elmB = t2; elmG = t2 + t3;
}
else if (elmH < 120) {
elmG = t1; elmB = t2; elmR = t1 - t3;
}
else if (elmH < 180) {
elmG = t1; elmR = t2; elmB = t2 + t3;
}
else if (elmH < 240) {
elmB = t1; elmR = t2; elmG = t1 - t3;
}
else if (elmH < 300) {
elmB = t1; elmG = t2; elmR = t2 + t3;
}
else if (elmH < 360) {
elmR = t1; elmG = t2; elmB = t1 - t3;
}
else {
elmR = 0; elmG = 0; elmB = 0;
}
}

elmR = Math.floor(elmR).toString(16);
elmG = Math.floor(elmG).toString(16);
elmB = Math.floor(elmB).toString(16);
if (elmR.length == 1) elmR = "0" + elmR;
if (elmG.length == 1) elmG = "0" + elmG;
if (elmB.length == 1) elmB = "0" + elmB;

elmH = elmH + rate;
if (elmH >= 360)
elmH = 0;

return '#' + elmR + elmG + elmB;
}

//]]>

</script>

See the Pen ujDoz by Seu Tutorial (@marcelost) on CodePen