In printing, true black is produced by a single black dye. Process black, however, is generated by overprinting the process colors magenta, cyan, and yellow. Theoretically, the combination of these three colored dyes should create a perfect black, but in reality, process blacks, which are also known as composite blacks, sometimes exhibit a greenish tint. To correct this problem, true black is often added to the printing process in order to ensure that quality blacks are produced in final images and text.