The value of a selected channel is graphed from 0 to 255 along the horizontal axis of the histogram. The meaning of the value depends on the selected channel:
Red, Green, and Blue values represent the amount of red, green, or blue in each color channel. A channel value of zero indicates the color is not present and a value of 255 indicates the color is at full strength. For example, if the graph has a spike at the value 128 for the Red channel, then there are a large number of pixels with a red value of 128 (or middle red).
Hue, Saturation, and Lightness values are a different representation of color than the Red, Green, and Blue channels. Hue is the actual color and the value is the number assigned on the color wheel; for example, red is zero, yellow is 43, green is 85, cyan is 128, blue is 170, and magenta is 212. (As a reference, look at the color wheel on the Jasc Color Picker.) Saturation is the level of grey added to the hue; zero is very grey and unsaturated and 255 is no grey and fully saturated. Lightness is the brightness of the hue; zero is no light (black), 128 is middle grey, and 255 is fully light (white). The lightness histogram has information similar to the greyscale histogram.
Greyscale values are levels of grey; zero is black, 128 is middle grey, and 255 is white. These represent the brightness or luminosity of an image. The distribution of grey values determines the contrast of an image.
Adjusting Individual Brightness Values
Adjusting the Brightness and Contrast in Unison
Adjusting the Brightness of the Highlights, Midtones, and Shadows
Adjusting the Lightness Levels
Manually Adjusting Brightness and Contrast