You’re probably used to digital images being in the RGB mode, where the full range of colors is generated with red, green and blue color ‘channels’. But most photo editing programs offer a color editing mode based around the HSL (Hue, Saturation, Lightness) color model, and this is where it gets really interesting.
Global HSL adjustments aren’t very useful. If you shift the global hue of an image it quickly looks wrong. The real strength of the HSL system is the way it lets you separate and edit individual colors.