When grayscale is selected, the resolved print color is gray shade arrived at by the formula 30% R + 59% G + 11% B, where R, G, and B are the red, green, and blue input color components.
Screening is a misnomer, and exists mainly for compatibility with AutoCAD plot style tables. Screening produces a "washed out" effect by adjusting each color component by the same amount toward white. The amount of adjustment is "100 - screening", so a screening value of 100 results in no change, while a screening factor of 0 yields pure white. A screening value of 50 would result in roughly the same hue and saturation of the original color, but increased lightness.