The formulas of §10.4.2: red = 1 − min(1, c + k), green = 1 − min(1, m + k), blue = 1 − min(1, y + k), and gray = 1 − min(1, 0.3 c + 0.59 m + 0.11 y + k), which §10.4.2.1 calls crude approximations.
Classic = 1