If you let light from the three bulbs shine through a hole in a card that is held an appropriate distance from the screen, you will see three separate patches of colored light on the screen, one from each lamp. (Make the hole large enough to get a patch of color you can really see.) If you move the card closer to the screen, the patches of light will eventually overlap and you will see the mixtures of each pair of colors.
When a red light, a blue light, and a green light are all shining on the screen, the screen looks white because these three colored lights stimulate all three types of cones in your eyes approximately equally, creating the sensation of white. Red, green, and blue are therefore called additive primaries of light.
Remove the object, turn off one of the colored lights, and notice how the color on the screen changes. Put the object in front of the screen again and notice the colors of the shadows. Move the object close to the screen until the shadows overlap. Notice the color of the combined shadows.
Make the room as dark as possible. Then turn on the three colored lights, aim them all at your white screen, and adjust the positions of the bulbs until you obtain the “whitest” light you can make on the screen.