paste on console and 🌈

FB.rainbow.CSSgradient.js

function repImg( tar ) {
    var div = document.createElement( 'div' );

    div.style.height = tar.height + 'px';
    div.style.width = tar.width + 'px';
    div.style.display = 'inline-block';
    div.style.backgroundColor = 'transparent';

    // http://stackoverflow.com/questions/9847580/how-to-detect-safari-chrome-ie-firefox-and-opera-browser
    var isFirefox = typeof InstallTrigger !== 'undefined';
    var isChrome = !!window.chrome && !isOpera;
    var isOpera = !!window.opera || navigator.userAgent.indexOf( ' OPR/' ) >= 0;

    var gradient;
    // https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient

    if ( isChrome ) {
        gradient = "-webkit-linear-gradient(top, #FE6648, #FEB144, #FEE142, #4AFE92, #43C2FD, #8D50FE)";
    } else if ( isFirefox ) {
        gradient = "-moz-linear-gradient(top, #FE6648, #FEB144, #FEE142, #4AFE92, #43C2FD, #8D50FE)";
    } else if ( isOpera ) {
        gradient = "-o-linear-gradient(top, #FE6648, #FEB144, #FEE142, #4AFE92, #43C2FD, #8D50FE)";
    } else {
        gradient = "linear-gradient(top, #FE6648, #FEB144, #FEE142, #4AFE92, #43C2FD, #8D50FE)";
    }

    div.style.backgroundImage = 'url("' + tar.src + '"), ' + gradient;
    div.style.backgroundBlendMode = 'multiply';
    div.style.backgroundSize = '100%';

    tar.parentNode.insertBefore( div, tar.nextSibling );

    tar.style.display = 'none';
}

function replaceAll() {
    var imgs = document.querySelectorAll( 'img' );
    [].forEach.call( imgs, ( function ( img ) {
        if ( !img.classList.contains( 'rainbow' ) ) {
            repImg( img );
            img.classList.add( 'rainbow' );
        }
    } ) );
}

var registerObserver = function () { // thanks dannvix
    var MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

    var throttle = ( function () {
        var timer_;
        return function ( fn, wait ) {
            if ( timer_ ) {
                clearTimeout( timer_ );
            }
            timer_ = setTimeout( fn, wait );
        };
    } )();

    var target = document;
    var config = {
        attributes: true,
        childList: true,
        characterData: true,
        subtree: true,
    };

    var mutationObserver = new MutationObserver( function ( mutations ) {
        var hasNewNode = false;
        mutations.forEach( function ( mutation, idx ) {
            if ( mutation.type == 'childList' && mutation.addedNodes.length > 0 )
                hasNewNode = true;
        } );
        if ( hasNewNode ) {
            throttle( function () {
                replaceAll();
            }, 500 );
        }
    } );

    mutationObserver.observe( target, config );
};

registerObserver();

FB.rainbow.js

function repImg( tar ) {
    var div = document.createElement( 'div' );

    div.style.height = tar.height + 'px';
    div.style.width = tar.width + 'px';
    div.style.display = 'inline-block';
    div.style.backgroundColor = 'transparent';
    div.style.backgroundImage = 'url("' + tar.src + '"), url("http://www.csie.ntnu.edu.tw/~4024s/img/rainbow.png")';
    div.style.backgroundRepeat = 'no-repeat, no-repeat';
    div.style.backgroundPosition = ' 0% 0%, 0% 0%';
    div.style.backgroundSize = '100%, 100%';
    div.style.backgroundBlendMode = 'multiply';

    tar.parentNode.insertBefore( div, tar.nextSibling );

    tar.style.display = 'none';
}

function replaceAll() {
    var imgs = document.querySelectorAll( 'img' );
    [].forEach.call( imgs, ( function ( img ) {
        if ( !img.classList.contains( 'rainbow' ) ) {
            repImg( img );
            img.classList.add( 'rainbow' );
        }
    } ) );
}

var registerObserver = function () { // thanks dannvix
    var MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

    var throttle = ( function () {
        var timer_;
        return function ( fn, wait ) {
            if ( timer_ ) {
                clearTimeout( timer_ );
            }
            timer_ = setTimeout( fn, wait );
        };
    } )();

    var target = document;
    var config = {
        attributes: true,
        childList: true,
        characterData: true,
        subtree: true,
    };

    var mutationObserver = new MutationObserver( function ( mutations ) {
        var hasNewNode = false;
        mutations.forEach( function ( mutation, idx ) {
            if ( mutation.type == 'childList' && mutation.addedNodes.length > 0 )
                hasNewNode = true;
        } );
        if ( hasNewNode ) {
            throttle( function () {
                replaceAll();
            }, 500 );
        }
    } );

    mutationObserver.observe( target, config );
};

registerObserver();