User`s manual

Reference: Appendix 125
Advantages & Disadvantages to UCR
Undercolor removal within a TAC constraint reduces the problem of printing four
solid layers of ink, one on top of the other, while each previous layer is still wet.
Reducing the ink coverage, TAC, improves the ability of the paper to firmly hold
each layer of wet ink.
Less ink means better control, and faster drying times. Also, replacing cmy ink
with the less expensive black ink has proven more cost efficient on long runs that
use more ink.
Also, blacks and neutrals that print black are not influenced easily by shifts in the
chromatic inks which cause color casts in the shadow tones. Separations produced
with greater UCR produce darker blacks which result in better shadow detail.
However, some printers don’t like high UCR because of on-press dot gain and
contrasty rosettes.
GCR
Gray component replacement is a variation on undercolor removal. The theory is
simple: Whenever amounts of cyan, magenta, and yellow are present in the same
color, that color has a gray component. Some or all of this gray component can be
printed with black ink while maintaining the original color appearances.
In contrast to UCR, GCR involves a more general color replacement, with black
being substituted over all colors.
The neutral center of both diagrams looks the same. With more GCR applied, less
color ink and more black ink produces the same color.