Your computer screen mixes color as light, and therefore follows additive color mixture rules. If you want to see some amazing animations of hue and value relationships, try going to this link, which will also take you to a good descriptive explanation of hues and primaries. For a more technical discussion of how the primaries combine to form colors in the eye, try this link to a scholarly paper, The Representation of Color Metrics and Mappings in Perceptual Color Space . There are many systems for classifying hue, developed so that researchers can measure and define color qualities, and so that designers, industry, and marketing people can communicate color ideas over distance. This link will introduce you to some of the most common of these color communication systems.
Complements are colors that are opposite one another on the hue circle.
Such opposite pairs can be compared in terms of their relative warmth and coolness. Warm-cool contrast of hue can cause images to appear to advance or recede. In this 15th century painting, for example, the warm reds of the man's doublet and his son's cap reinforce the cues of placement to make these figures seem very close. On the other hand, the cool tones of the sea and sky suggest great distance. Afterimage is another, more specific definition of complements consisting of a stimulus color and its physical opposite generated in the eye by exposure to the stimulus color.