Can you point to some examples? I have been scanning paper money for 10+ years and have no idea what you are talking about.
Oh, this is usually caused by the (dirty) glass within the scanner itself. They are likely using an old scanner and need to clean the glass ( on both sides ) but I don't recommend most people to crack open their scanner to clean the side of the glass on the underside where the scanner rolls to scan the pictures. Also, when cleaning, avoid using cleaning solutions with ammonia.
It is unrelated to dirt. This is a case of chromatic aberration in low quality or old scanners optics. Read this for more http://en.wikipedia.org/wiki/Chromatic_aberration
I'm sure it depends on the scanner as well. I've seen this on older HP scanners, simply cleaning the glass fixed the rainbow that would show up.
Or maybe you just placed the object under a slightly different angle, which solved the problem. Dirt usually adds ugly stains or dots, not rainbow colors. I think the color effect might also happen when a small amount of water or humidity presents on scanner glass or, like in this case, inside the plastic banknote sleeve.
