The Levels command was used to correct colors of the original image. For reference, the histogram and Input Levels from the Levels dialog box are inset with each step. Starting with the original image, highlights were first adjusted using the white slider. Next, midtones were adjusted using the gray slider. Then, the shadows were adjusted using the black slider. Finally, the midtones were readjusted to add more color depth. Compare the results of manually adjusting levels with the results of choosing the Auto Levels command.
Original image
Highlights adjusted
Midtones adjusted
Shadows adjusted
Midtones readjusted
Auto levels