Color Theory Basics - Undercolor Removal & Gray Component Replacement

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Under Color Removal UCR and Gray Component Replacement GCR are techniques used for making color separations work better in the printing process. The concepts and techniques of UCR and GCR within the color separation process are complicated. To begin to understand how UCR and GCR techniques work in the color separation process, two basic image areas have to be identified. They are neutral and color areas on images.

Neutral areas are identified by neutral gray areas such as white highlights gray midtones and black shadow areas. On the other hand, color areas are colorful hues of a reproduction. Color areas are comprised of six basic colors red, green, blue, cyan, magenta, and yellow. Each color ranges from light colors to pure hues.

Neutral and color image areas have their own unique relationship to CMYK inks in the printing process. Note charts on page 6 about neutral gray and how to create color values on page 9.

The purpose of the UCR technique is to compensate for a printing problem called ink trapping. Poor ink trapping causes cyan, magenta, yellow and black inks to dry improperly when they are overprinted. Poor trapping can ruin the color reproduction.

Under Color Removal

The UCR technique affects only the neutral shadow areas of the reproduction. It is applied by removing cyan, magenta and yellow ink and replacing black ink in those areas. This technique allows less ink overall to be applied to the paper in the shadow areas of the reproduction. UCR requirements are identified by total printing density in percent dot values. The measurements are taken from the darkest neutral shadow areas of the reproduction.

Comparing Density Values

The total ink density of a color separation is based on adding up the total value of the four-color halftone inks at their darkest neutral shadow point in the color separation. Comparing the total printing density of halftone dot values between a traditional full range color separation without UCR to a color separation with UCR, makes differences apparent.

Gray Component Replacement

The GCR technique interacts with two areas of the reproduction, the neutrals and colors areas. One benefit of GCR is to permit color areas to print easier. Another plus for GCR is when it is adjusted correctly it allows color areas to print cleaner, offering brighter colors in the final printing than with traditional color separations.

GCR affects neutral shadow areas, exactly like UCR. It also affects color areas, by removing amounts of cyan, magenta and yellow ink that are equal to gray. The gray component is replaced with black ink.

GCR is not a new technique. Only in the mid 1980's did it become popular. Even though sav-ing ink with UCR and GCR is possible, two basic arguments surround the benefits and trade offs of GCR. Some printers enjoy the GCR technique because of the easy press make ready and printing. Print buyers dislike it because they claim shadow areas of the reproduction suffer due to a loss of ink density removed in the process. Fortunately, the problem of density loss is overcome by the technique of Undercolor Addition (UCA). UCA is used in conjunction with GCR, and can be adjusted to any level for ink trapping problems. The UCA technique adds back cyan, magenta and yellow ink, to the neutral shadows of the color separation. This makes the neutral shadow areas potentially like traditional color separation, while getting the benefits of the GCR applied to the color areas of the color reproduction.

Following are some examples of CMYK values for colors with and without GCR.

Red Colors w& wo GCR	Cyan Colors w& wo GCR
Green Colors w& wo GCR	Magenta Colors w& wo GCR
Blue Colors w& wo GCR	Yellow Colors w& wo GCR

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