Combining the answers from Get a CSS value with JavaScript and How to get a pixel's x,y coordinate color from an image?, I was able to simulate what you are looking for: JSFiddle
<html>
<head>
<style type="text/css">
#test {
background-image: url('http://jsfiddle.net/img/logo.png');
background-color: blue;
background-repeat: no-repeat;
height: 30px;
width: 100%;
}
#hidden {
display: none;
}
</style>
<script type="text/javascript">
var div = document.getElementById("test");
var style = window.getComputedStyle(div);
div.onmousemove = function(e) {
var path = style.getPropertyValue('background-image');
path = path.substring(4, path.length-1);
var img = document.getElementById("i1");
img.src = path;
var canvas = document.getElementById("c1");
canvas.width = img.width;
canvas.height = img.height;
canvas.getContext('2d').drawImage(img, 0, 0, img.width, img.height);
var pixelData = canvas.getContext('2d').getImageData(event.offsetX, event.offsetY, 1, 1).data;
var values = document.getElementById("values");
values.innerHTML = 'R: ' + pixelData[0] + '<br>G: ' + pixelData[1] + '<br>B: ' + pixelData[2] + '<br>A: ' + pixelData[3];
};
</script>
</head>
<body>
<div id="test"></div>
<div id="hidden">
<img id="i1" src="" />
<canvas id="c1"></canvas>
</div>
<div id="values"></div>
</body>
</html>
I retrieved the computed style (var style = window.getComputedStyle(div);) outside of the mouse move function for performance reasons, but if the background image is going to change dynamically, then it may need to be moved into the function.
There are some possible browser constraints for using getComputedStyle.
SCALING
You could try editing the code to adjust for the scale:
var h = parseInt(style.getPropertyValue("width")),
w = parseInt(style.getPropertyValue("height"));
var img = document.getElementById("i1");
img.src = path;
var canvas = document.getElementById("c1");
canvas.width = h;
canvas.height = w;
canvas.getContext('2d').drawImage(img, 0, 0, w, h);
This also includes a change to the CSS: JSFiddle
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