Colors through photographing (RGB) are different from colors printed on paper (CMYK). Also, ink printed on different paper will also varies a lot. As a result, in order to print the colors on the specified paper correctly, we need to adjust the curves matching the colors with the materials, and make the RGB to CMYK switch is accurate. Plus, some images do not have great colors themselves, so by color retouching, we could make the images look better.