The following code mounts an object containing the histogram and retrieves the most common color using an invisible% element:

//carrega uma imagem
var img = new Image();
img.src = ...

//cria um canvas invisível
var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
var context = canvas.getContext('2d');

//desenha a imagem no canvas
context.drawImage(img, 0, 0);

//recupera vetor de cores
var map = context.getImageData(0, 0, img.width, img.height).data;

//monta histograma
var hex, r,g,b; //,alpha;
var histograma = {};
for (var i = 0, len = map.length; i < len; i += 4) {

    //recupera componentes de um ponto
    r = arredonda(map[i]);
    g = arredonda(map[i+1]);
    b = arredonda(map[i+2]);
    //alpha = map[i+2]; //ignora canal alpha

    //valor em hexadecimal
    hex = rgbToHex(r, g, b);

    //adiciona no histograma ou incrementa se já existir
    if (histograma[hex] === undefined) {
        histograma[hex] = 1;
    } else {
        histograma[hex]++;
    }
}

//recupera cor mais comum
var corMaisComum = null;
var frequenciaCorMaisComum = 0;
for (var cor in histograma) {
    if (frequenciaCorMaisComum < histograma[cor]) {
        corMaisComum = cor;
        frequenciaCorMaisComum = histograma[cor];
    }
}

console.log(corMaisComum);

The histogram is nothing more than a map type where the key is the color in hexadecimal notation and the value is the frequency with which it occurs in the image.

In this algorithm, I used the canvas function to retrieve the color to round it to the nearest multiple of 5. Thus, slight variations in color will not disrupt the final result.

Note that with p histogram we can not only get the most common color, but also the first% w / o% most common colors. For this we would just have to create a vector with colors sorted by frequency.

The above code can be seen running on the jsdo.it site, but I'll post the same complete , if it becomes inaccessible one day:

//"arredonda" o número para o múltiplo de 5 mais próximo
//isso adiciona uma certa tolerância para os tons próximos
function arredonda(v) {
    return 5 * (Math.round(v / 5));
}

function componentToHex(c) {
    var hex = c.toString(16);
    return hex.length == 1 ? "0" + hex : hex;
}

function rgbToHex(r, g, b) {
    return "#" + componentToHex(r) + componentToHex(g) + componentToHex(b);
}

//carrega uma imagem
var img = new Image();
img.src = 'http://jsrun.it/assets/8/O/3/U/8O3Ux.jpg';
img.onload = function() {

    $(document.body).append(img);

    //cria um canvas invisível
    var canvas = document.createElement('canvas');
    canvas.width = img.width;
    canvas.height = img.height;
    var context = canvas.getContext('2d');

    //desenha a imagem no canvas
    context.drawImage(img, 0, 0);

    //recupera vetor de cores
    var map = context.getImageData(0, 0, img.width, img.height).data;

    var hex, r,g,b; //,alpha;
    var histograma = {};
    for (var i = 0, len = map.length; i < len; i += 4) {

        //recupera componentes de um ponto
        r = arredonda(map[i]);
        g = arredonda(map[i+1]);
        b = arredonda(map[i+2]);
        //alpha = map[i+2]; //ignora canal alpha

        //valor em hexadecimal
        hex = rgbToHex(r, g, b);

        //adiciona no histograma ou incrementa se já existir
        if (histograma[hex] === undefined) {
            histograma[hex] = 1;
        } else {
            histograma[hex]++;
        }
    }

    //recupera cor mais comum
    var corMaisComum = null;
    var frequenciaCorMaisComum = 0;
    for (var cor in histograma) {
        if (frequenciaCorMaisComum < histograma[cor]) {
            corMaisComum = cor;
            frequenciaCorMaisComum = histograma[cor];
        }
    }

    console.log(corMaisComum);

    //adiciona um div como exemplo
    $(document.body).append(
        $('<div>').css({
            'background-color': corMaisComum,
            'width': '200px',
            'height': '200px',
            'border': '1px solid #000'
        })
    );

};

The visual result looks like the image below:

You can use the Canvas technology that is contained in browsers that support HTML5 (currently most), and with it you can do a JavaScript function that takes your image as a parameter and checks from 5 to 5 pixels which is the given color, and checks the entire image, and then the predominant color is set via R, G, B (red, green, blue) at the bottom of your page.

HTML Exemplary would be:

<span> Background é setado para a cor predominante desta imagem: </span>
<img id="i" src="data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCAgMCAgMDAwMEAwMEBQgFBQQEBQoHBwYI
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BQYEA//EADkQAAEDAwIFAgQDBAsAAAAAAAECAwQABREGBwgSITFBUWETInGhCYKRFFKB
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Note: Converted your image into base64 :)

And the function that makes magic would be this:

    var rgb = getAverageRGB(document.getElementById('i'));
    document.body.style.backgroundColor = 'rgb('+rgb.r+','+rgb.g+','+rgb.b+')';

function getAverageRGB(imgEl) {

    var blockSize = 5, //checa a cor à cada 5 pixels
        defaultRGB = {r:0,g:0,b:0}, // para browsers com incompatibilidade
        canvas = document.createElement('canvas'),
        context = canvas.getContext && canvas.getContext('2d'),
        data, width, height,
        i = -4,
        length,
        rgb = {r:0,g:0,b:0},
        count = 0; //zera o contador

    if (!context) {
        return defaultRGB;
    }

    height = canvas.height = imgEl.naturalHeight || imgEl.offsetHeight || imgEl.height;
    width = canvas.width = imgEl.naturalWidth || imgEl.offsetWidth || imgEl.width;

    context.drawImage(imgEl, 0, 0); //desenha a imagem cria o canvas

    try {
        data = context.getImageData(0, 0, width, height);
    } catch(e) {
        /* gera um erro de segurança, se a imagem for de dominio diferente */alert('x');
        return defaultRGB;//retorna  a cor padrão
    }

    length = data.data.length; //tamanho do array dos dados da imagem

    while ( (i += blockSize * 4) < length ) {
        ++count; //incrementa o contador previamente zerado
        rgb.r += data.data[i];
        rgb.g += data.data[i+1];
        rgb.b += data.data[i+2];
    }

    // ~~ usado para arrendondar valores para baixo
    rgb.r = ~~(rgb.r/count);
    rgb.g = ~~(rgb.g/count);
    rgb.b = ~~(rgb.b/count);

    return rgb;

}

Example running on JSFiddle with your image.

Important:

The function is not my own, here is the Reference response in SOEN .