Introduction

Although we usually think of a single band of an image as being displayed as shades of gray (intensities of 0 to 255 for 8 bit data), one can take a single band and assign colors to given ranges of digital values.

RGB

Red, green and blue are called additive colors (think of mixing paint) and most displays are produced this way. In remote sensing, the colors are not truly added in that the screen of the computer monitor is actually composed of pixels which consist of three segments which are illuminated by the red, green, and blue guns of the system. These segments are so small as to merge and give the full range of colors. For 8 bit data, the combinations of 256 intensity values for three colors lead to millions of colors.

Subtractive colors

Yellow, magenta, and cyan. These colors are mixtures of the additive colors (yellow - red & green, magenta - blue & red, cyan - blue & green). We use the term subtractive because we think of light passing through a filter such as a yellow one where blue is subtracted and red and green remain.

IHS

Intensity - brightness of the color; Hue - dominant wavelength; Saturation - pureness of the color (pastels have intermediate saturation values). One set of the several transformation equations that are used are as follows:
I = R+G+B; H=G-B/I-3B; S=I-3B/I

A common procedure to increase the richness (saturation) of the color in an image is to apply the IHS transform to the data, stretch the saturation values, and return to RGB space and view the image.