Deke McClelland explains what the Levels and Histogram commands can do in Photoshop in this video tutorial. Levels allows you to adjust the luminance levels allowing you to adjust the contrast and colors within each color channel of an image. Thus, Levels allows you to adjust the individual colors that can be found within a gradient. This command is very similar to the curves command but to a less powerful extent. It is also more efficient than the curves command. The Histogram is another color-related command. Essentially, the histogram is displayed in a bar graph image and represents the image's tone and quantity. In totality, it represents the tonal density of a picture. Lots of black on the left portion of the graph indicates that there are several dark parts in the picture. Lots of dark parts on the right side of the graph are indicative of many light parts in the picture. Use these commands to analyze an image before actually editing it.