Sometimes, a specific color must be reproduced exactly the same every time it is printed (for example, a corporate logo). If that color were created through the standard application of 4 separate colors of ink (Cyan, Magenta, Yellow, and Black) during the 4-color printing process, it would exhibit very slight color variance between different print runs.
The only way to make sure a specific color looks exactly the same every time it is printed is to use what is known as a spot color. When using a spot color, the ink is pre-mixed to match the specified color, and is applied via its own printing plate. The most common way of specifying the exact color to be matched is by using the Pantone Color Matching System:
Because these colors can be made on their own without needing to consist of just Cyan, Magenta, Yellow, and Black, the color range of Pantone inks is far wider than the standard CMYK gamut, and it can include specialty inks such as metallic or fluorescent colors. Pantone colors are usually indicated by referring to their code: for example PMS 3245 is a bright, seafoam green.
Using spot colors always adds significant cost to an order, since they require prepress, printing plate manufacturing, press setup, and a press run for the extra color of ink to be applied. Metallic and specialty inks are also more expensive than standard 4 color process inks. However, when you need an exact color for branded materials like logos, or a specialty color that can’t be created by CMYK, they’re a great option.