For Firefox, there is the excellent ColorZilla extension that does exactly that. However, it's FF only of course, and I would really like to deliver the tool along with the CMS.
A dutch developer has had the very clever idea of using a combination of Ajax and PHP's
http://some-other-host/yourPassword.png. You can only tell the color of the pixel under the mouse if either the mouse is over a canvas or an image element of the same domain (or an image element of another domain which is served with an
Access-Control-Allow-Origin: * header). In the case of the canvas, you would do
canvasElement.getContext('2d').getImageData(x, y, 1, 1).data. In the case of the images, you would have to draw them to a canvas with:
var canvas = document.createElement("canvas"); canvas.width = yourImageElement.width; canvas.height = yourImageElement.height; canvas.getContext('2d').drawImage(yourImageElement, 0, 0);
And then just use the previous method explained for canvases. If you must be able to convert to various representations of color values, try my color.js library.
Also, you're never going to be able to support IE <9 (that's assuming that IE9 supports canvas) and using Flash won't help as it can't read the pixel data of the document either.