Apple Color 1.0 - Manual (Page 231)

Chapter 10 Secondaries 231
For example, if you add four control points to the Saturation curve to lower the green-
through-blue range of the curve, you can smoothly desaturate everything that’s blue
and green throughout the frame, while leaving all other colors intact.
One of the nicest aspects of these controls is that they allow for extremely specific
adjustments to narrow or wide areas of color, with exceptionally smooth transitions
from the corrected to the uncorrected areas of the image. In many instances, the
results may be smoother than might be achievable with the HSL qualifiers.
Another key advantage these controls have over the HSL qualifiers is that you can
make simultaneous adjustments to discontiguous ranges of hue. In other words, you
can boost or lower values in the red, green, and blue areas of an image while
minimizing the effect of this adjustment on the yellow, cyan, and magenta portions of
the image.
Before After
Sat curve adjustment