http://psych.wisc.edu/braun/406/Outlines/Color2.htm

Retinex Theory

• A theory of color constancy that is based on the comparison of the lightness readings of surfaces.
• According to this theory, a colored scene is analyzed into 3 lightness records.
• These lightness records give a measure of how much of the available red, green, and blue light a surface reflects.
• These lightness records can be thought of as equivalent to viewing the scene through three monochromatic (single-colored) filters, one at a time – one red, one green, one blue.
• For example, when a scene is viewed though a red filter, one sees an array of reds ranging from very light to very dark. The lightest areas of a scene viewed through a red filter reflect a lot of long-wavelength light and the darker areas reflect little of the long-wavelength light. So red areas in the scene will be the lightest when viewed through a red filter. In contrast, when a scene if viewed though a green filter, green areas in the scene will be the lightest.
• When comparing the output of the 3 lightness records, the lightest part of all three records will be perceived as white, since white reflects more of any wavelength than colored surfaces

http://people.msoe.edu/~taylor/eisl/land.htm

Practical Example of how to make a two-colour full-colour picture