UCR -Removes primary colors from deep shadow areas and replacing them with black. This saves the amount of (expensive) colored ink used and avoids very high ink weights which may cause setoff and marking.

• UCR makes use of a total ink limit to control the amount of ink in the overprints. • For web offset (which is especially prone to ink smearing or marking) the total ink limit is generally set to 250%, while for sheetfed offset 300 to 350% is acceptable. We illustrate this in Photoshop.
• UCR reduces possible contrast and hence shadow detail, but stabilizes color balance in shadows.

GCR -Removes primary colors throughout the entire tonal range. This greatly reduces the amount of colored ink used and increases the control of color balance.

• GCR makes use of a percentage of gray removal.
• 50% gray removal corresponds to the default “medium” black generation setting in Photoshop.
• The GCR mode in Photoshop also supports undercolor addition (UCA) which adds color to deep shadows to extend the density range. We illustrate GCR.
• GCR is used extensively in flexo.
• When selecting PMS from custom colors in Photoshop, the recommended separations often consist of two colors plus black. These can be converted to the desired amount of gray if less than 100% GCR is desired.