Apple Color 1.5 - Manual (Page 262)

The HSL controls work as a chroma keyer. By selecting ranges of hue, saturation, and
lightness, you create a matte that is then used to define the region to which corrections
are applied. Everything outside the matte remains unaffected (although you can also
specify which portion of the matte you want to adjust, the inside or the outside).
Original image HSL qualifier settings
Matte Corrected image
The HSL Qualifier controls always sample image data from the original, uncorrected image.
This means that no matter what adjustments have been made in the Primary In room,
the original image values are actually used to pull the key. For example, even if you
completely desaturate the image in the Primary In room, you can still pull a chroma key
in the Secondaries room.
Tip: It is not necessary to use all three qualifiers when keying on a region of the image.
Each qualifier has a checkbox and can be turned on and off individually. For example, if
you turn off the H (hue) and S (saturation) controls, you can use the L (lightness) control
by itself as a luma keyer. This is a powerful technique that lets you isolate areas of an
image based solely on image brightness.
Creating Fast Secondary Keys Using the HSL Eyedropper
The eyedropper, at the top-left corner of the Basic tab, provides a quick and easy way to
sample color values from images you’re correcting.
262 Chapter 10 The Secondaries Room