This adjustment is of great use. By mixing the RGB value of the Red, Green, Blue channels which are called Source Channels, users are able to create special color adjustments that cannot be easily generated by other adjustments. For example, it is possible to change the image from a green-spring tune to a golden-autumn tune with this feature. Although the three channels are tuned separately, users always see the accumulated overall impact.

A dialog pops up upon choosing the Channel Mixer. Select a channel in the Output Channel box, the value of this channel will be set to 100% and values of other channels are 0% by default. To decrease the contribution of one channel to the output channel, drag the corresponding slider to the left, and vice versa. The Total means the total value of the source channels. The Constant is to adjust the grayscale value of the output channel. A negative value means darker, and a positive value means lighter. The Monochrome option changes the image to a gray one, which helps to create a high-quality gray-scale picture.

This adjustment is used to balance the image colors, to remove unwanted color casts, and to correct over-saturated/under-saturated colors. This command changes the overall mixture of colors in an image.

Color Levels on the top of this panel display the color balance values for R, G, B channels. There are three sliders, Cyan-Red, Magenta-Green, Yellow-Blue. Drag a slider toward a color if you want to increase its impact in an image, or drag a slider away from a color if you want to decrease its impact. In the Tone Balance field, choose Shadows, Midtones or Highlights to get the tonal range in which you want to implement the changes. Select the Preserve Luminance option to keep luminance unchanged while changing the color.

This adjustment is to adjust the entire tonal range of an image, or to make precise adjustments to individual color channels. It is the most important and convenient tool in color and tonal adjustments. 

In Curves adjustment, the tonal range is represented as a straight diagonal line with the X-axis as input levels and the Y-axis as output levels. Change the shape of the curve to adjust the tonal and color of an image. Move the curve upward/downward to lighten/darken an image. The steeper/flatter parts of the curve correspond to areas of higher/lower contrast. Choose a Channel, Red, Yellow or Green, to adjust the color balance.

This adjustment is to correct the tonal range and color balance of an image by adjusting the intensity levels of image shadows, mid-tones, and highlights.

In the Levels dialog box, the outer two of the three Input Levels sliders, map the black and white points to the settings of the Output Levels sliders. By default, the Output Levels sliders are at level 0 and level 255, respectively. With the Output Levels sliders in the default positions, a user could adjust the parameters as follows.

Move the black input slider to A so as to map all the pixels with their levels equal or lower than level A to level 0, i.e., discarding black details. Move the white input slider to C to map all the pixels with their levels equal or above level C to level 255, i.e., discarding the white details. The remaining levels are redistributed between levels 0 and 255. Such operations increase the tonal range of the image and the overall contrast.

The middle input slider is to adjust the gamma value in an image, as well as the intensity value of the middle range of gray tones. Moving it to the left makes the image lighter. This adjustment maps a lower level up to the mid-point level between the Output Levels sliders, and here it is 128. The shadows expand to fill the tonal range from 0 to 128 while the highlights are compressed. 

Tips: Drag the Output Levels sliders to set the output tonal range, to which the Input Levels are mapped.

This adjustment manually detects the white balance reference point in the image, and adjusts white balance on such a basis. 

For example, pictures taken indoors under incandescent light are of a yellowish color, pictures taken indoors under fluorescent are of a greenish color, and pictures taken outdoors in sunlight shadows are of a bluish color. 

When choosing this adjustment, a suction tool emerges on the workspace, with which you can click to choose a reference pixel which should be white.

This adjustment automatically detects the white balance reference point in an image, and then adjusts white balance on this basis. Click to apply it.

For example, pictures taken indoors under incandescent light are of a yellowish color, pictures taken indoors under fluorescent are of a greenish color, and pictures taken outdoors in sunlight shadows are of a bluish color. Then the Auto White Balance can pick up “white” pixel automatically and correct the color cast.