This applet allows you to subtractively mix colors. The beam starts out as white. You may then take out the blue, green, and red components by altering the scroll bars or text boxes. Notice that the filter for removing the blue is yellow, to remove green it's magenta, and to remove red it's cyan. The number at the bottom represents the amount of red, green, or blue light removed - 1 means 100% of the specific color and 0 means 0% of the specific color.
If the colors of this applet appear odd to you, check the color calibration page.