Cesium Sandcastle Pixel Picker

sandcastleTest.html

<style>
    @import url(../templates/bucket.css);
    .point {
        border: 1px solid red;
        width: 5px;
        height: 5px;
        position: absolute;
        top: 197px;
        left: 197px;
    }
</style>

<div id="cesiumContainer" class="fullSize"></div>
<div id="toolbar"></div>
<div class="point"></div>

sandcastleTest.js

var viewer = new Cesium.Viewer('cesiumContainer');
var pixels = new Uint8Array(4);

function getColor(x, y) {
    var gl = viewer.canvas.getContext('webgl');
    gl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixels);
    console.log(pixels);
}

Sandcastle.addToolbarButton('Get Color at 200, 200', getColor.bind(null, 200, 200));