Just place a mark anywhere on the suspect bill. If the mark stays amber, the bill is genuine U.S. currency; if the mark turns black, the money may be counterfeit. What is going on? Ordinary paper contains starch, while U.S. currency paper does not. The pen ink contains an iodine-potassium iodide solution that when combined with starch, produces a characteristic blue complex. If no starch is present, the color is amber. Amber markings will fade shortly after application.