By the look of the paper of your stamp, it has been badly stored in the past. The gold color is a result of fading of certain pigments or chemical changes ("changeling") or a combination of both. I think it is mostly fading as I've seen lilac/purple printing do pretty much the same thing when left out in the sun.
Unfortunately, I don't have an example to show, but see the surcharged version here:http://goscf.com/t/18758
While the image is too bright, you can see a change of color happening from bottom to top, on its way to turning more yellowish.