If, for example, we had a bill colored with light blues and greens, we might want sharp, striking black text to denote the value of the currency, as opposed to white text, which may be too light to be seen, or a blue, which may be too similar to the rest of the bill to be noticed. These two problems relate to intensity and saturation. Intensity is the brightness of a particular color. The difference between dark blue and light blue, for example, is a difference in intensity. The human eye has an easier time distinguishing between two colors of different intensity.
Saturation is the strength of the color.
Colored bills are also more difficult to counterfiet, since an additional production process must be used to colorize the bills. Since we are redesigning the entire bill, we must also consider motivations for keeping or changing the current size of US paper currency. We decided to keep the current size, since it is an ideal size for handling and storing. Our options would be a more square bill, or a more elongated bill. Although it is not a psychological issue, square bills would be difficult to give to machinery, since they could be inserted up to eight different ways.