Color grading is the process of improving the appearance of an image for presentation in different environments on different devices. Various attributes of an image such as contrast, color, saturation, detail, black level, and white point may be enhanced whether for motion pictures, videos, or still images.
Need a visual representation of color grading? Look no further than this image:
To learn the basics of color grading, look at this short video from filmmaker Justin Odisho:
To learn more about color and its effect on film, the DSLR Guide has a great video for you:
Learn more about color in general, with this great video on Color Theory by Cinematic J: