4-color black is made of a mixture of cyan, magenta, yellow, and black that looks deep and crisp and visually appealing. Flat black is made of 100% black (as a CMYK Value: C = 0%, M = 0%, Y = 0%, K = 100%). Flat black is requested for all text within a book, as well as barcode images. This ensures that there will be no registration issues in your book’s interior or barcode.