The paper or card that the diamond is laid upon when its color is being checked is called colored paper. When checking the color of a polished diamond, it is important that there are no ambient influences which might alter one's impression of a diamond's true color, such as: the color of the room, the work table, or even the clothes of the person checking the color.
In order to prevent ambient colors from affecting the test, the diamond must be checked on a plain white background, with no gloss, and with no yellowish hues, close to a neon light bulb, filtering the spectrum of colors. The easiest way to achieve these conditions are to fold a piece of white paper into a 'V', with the diamond laying in the resulting crease, with the table facing down, and the inspector looking at the body of the diamond through its pavilion.The upper side of the folded piece of paper blocks the background colors and allows one to accurately observe the true color of the diamond. To make sure that the diamond doesn't fall off the paper, the paper is folded and the diamond is laid on the inner crease. If one wishes to achieve a highly accurate color grading, a Master Stone is laid next to the diamond.
The color paper or color card is especially important when inspecting large diamonds, and therefore all diamond dealers have these. Therefore, some use it for advertising purposes. The advert is printed on the outer sides, so that when the card is folded, it does not appear in the area that is used to inspect the diamond. It has turned into a kind of business card that is used to check for color, and that's why it is called a 'color card'.