The screen colors arise from the relative intensities of the red, green and blue components of the pixels (look at the screen through a magnifying glass!). The intensity values range from 0 to 255, thus rgb(0,0,0) is black, rgb(255,255,255) is white, and rgb(255,0,0) is red. In total, there are 256 x 256 x 256 = 16777216 different colors available.
Here a color is chosen randomly every 1.5 second. Stop will show the values for the actual color.