Secondary Color Correction Primer - The Art and Technique of Digital Color Correction - page 192

Fig. 5.41 The shape of the vignette is hard to see since it sits right on the horizon.
The darker overall shade of my grass correction helps focus attention up
towards the ultralight.
Once the color of the of the grass is complete, switch the second-
ary control from “inside” to “outside.” With the sky, I started working
on the contrast first. I brought the master lift down a lot . This popped
the ultralight against the sky because it's the only really dark element
in the top half of the image. I brought the gain up so that the clouds
would “read” better against the blue of the sky and brought the gammas
down a bit to stretch the contrast between the sky and the clouds even
further. Then I pulled the midtones towards blue pretty severely. The
midtones only really affected the blue sky (remember that the grass is
being “protected” from the midtone correction by the spot correction).
Then I pulled the highlights—which was basically just the clouds—away
from blue so that the clouds would be more cleanly balanced towards
white ( Figure 5.42 ) .
The result is something that could not be executed in a primary
color correction, which is pretty obvious when you see the wildly
different corrections that went into the top and bottom of the image
( Figure 5.43 ) .