Apple Color 1.5 - Manual (Page 240)

To make adjustments to a Luma curve
1 Before making any actual adjustments, pin down the midtones and shadows of the image
by adding a control point to the curve without moving it either up or down.
Adding control points to a portion of a curve that you don't want to adjust, and leaving
them centered, is a great way to minimize the effect of other adjustments you're making
to specific areas of an image. When you add additional control points to adjust the curve,
the unedited control points you placed will help to limit the correction.
Tip: When adding multiple control points to a curve, you can use the grid to identify
where to position parts of a curve you want to be at the original, neutral state of the
image. At its uncorrected state, each curve passes through the diagonal intersections of
the background grid.
2 To make the actual adjustment, drag the white point at the upper-right corner down to
darken the sky.
You want to make sure that you don't drag the new control point down too far, since it's
easy to create adjustments that look unnatural or solarized using curves, especially when
part of a curve is inverted.
240 Chapter 9 The Primary In Room