The Hue Clock uses our knowledge of how to read a dial clock to provide a quick visual readout of the hue and saturation of a color.   The position of the hand easy to remember, verify, and adjust.

By remembering the "time" associated with a particular color, you can quickly and accurately judge the correctness of certain colors, including (see figure 1) flesh tones, foliage, and sky.

Even without special knowledge of the correct hue of an object, it is possible to quickly sweep over an object to check for any systematic hue variations. In figure 2, the sky has more blue near the top of the image, becoming less saturated, and more cyan in color as you move toward the bottom of the image.

Mixed lighting affects almost any outdoor subject, with the amount of blue increasing in the shadows. Figure 3 shows a photograph of a hawk taken in a common mixed lighting situation: direct sunlight combined with blue sky light. Using the hue clock it is easy to check for a change in hue angle, and take corrective action. 

With several samples in place, it often a matter of moving the dark endpoint of the blue curve horizontally to make all  the hue clocks show very close to the same reading. Naturally, instead of creating multiple samples, you may choose to drag a single sample around the image, and watch how the hand moves.

Flesh tones should show a hue angle of approximately 8-15 degrees, or an hour hand position of 12:30 on the hue clock. Think mid lunch hour.  Figure 3 shows a flesh tone from an uncorrected digital camera image - at 10 o'clock, it is much too blue, more of a violet color than a real flesh tone.

Figure 4 shows a normal flesh tone, in this case corrected via a pinning operation, showing a clock time of about 12:15.  The length of the clock hand is significant - flesh tones should be about the length of a clock's hour hand. Less than that, and the color turns gray. More, and your people will take on an overly orange color.

Instead of pinning, you may use curves manually to bring the Hue clock where it needs to be - just make sure when you are done that the length and position of the Hue clock is approximately that of an hour hand at about 12:30. 12 o'clock is generally too red of a skin tone, and 1 o'clock is too orange/yellow.

Surprisingly, although the hand may point anywhere between 12 and 1 o'clock, the race, or overall darkness or lightness of an individual's skin has very little effect on the position and length of the clock hand.

• The small pin icon in the bottom right corner of the palette is used to temporarily disable the pinned color, so that it does not affect the image. Toggling this pin in and out allows you to quickly determine how a given pinned sample changes the curves.
• With the pin "toggled" out, the sample will be retained when you click the Reset button in the curve window.
• You may always double click on the palette to use Photoshop's color picker to modify the sample's targeted color value.

Figure 5 shows hue clock positions for three different variations on sky color. Most people would know, simply by looking, that something is wrong with the sky on the far left, they might settle for the almost pure cyan sky in the right section of the image.

Pure blue is located at 8 o'clock on the color wheel, and this is reflected in the center sample, . The sky on the left has too much red, and the hue clock's hand has rotated toward 12 o'clock. The rightmost image has had green added, and the hue clock shows a time of about 7 o'clock.

The relatively gray granite rock in the lower part of the image shows an obvious red and green color cast for the left and right images, respectively. Neither of these would be acceptable as normal looking stone colors, but the sky colors are more subtly off, and the hue clock is a good visual indicator of this problem, and what direction the clock needs to be rotated to fix it.

As a practical matter, if I were actually correcting the above image, the gray neutral color of the granite would be a more reliable indicator of a color cast than any problems with the sky. But, when such an indicator is absent, or when other manipulations have changed the sky color, remember that, for sky, the hue clock should be at or near 7:30.

Next to flesh tones, perhaps we humans are most sensitive to shades of green, and indeed, most of the human sensory color space is devoted to green. Add a bit of yellow to a plant color, and most of us will immediately sense that the plant is suffering some sort of health problem. Add too much blue or green, and the plant takes on an unpleasant, poisonous appearance.

Our reactions, no doubt, go back to prehistory, when eating the wrong thing, or walking all day to the wrong colored hillside, could be a matter of life or death.

In figure 6, the grass, hedge, and even the very dark cypress evergreen plant in the upper left of the image show a very similar hue and saturation value. The darker leaves of a camellia plant are the greenest of all, near 3 o'clock, right between yellow and green.

Experiment on your own images, and you will find that plant colors start to look bad when they approach pure green or yellow. With very dark colors, the exact hue becomes less important, but a "wrong" green still indicates problems with the rest of the image.