when is a red shadow formed

A red shadow is never formed, the shadow is always black

With blue, red and green lights you can make shadows of seven different colors: blue, red, green, black, cyan (blue-green), magenta (a mixture of blue and red), and yellow (a mixture of red and green). If you block two of the three lights, you get a shadow of the third color: Block the red and green lights, for example, and you get a blue shadow. If you block all three lights, you get a black shadow. And if you block one of the three lights, you get a shadow whose color is a mixture of the two other colors. If the blue and green mix, they make cyan; red and blue make magenta; red and green make yellow.
If you turn off the red light, leaving only the blue and green lights on, the lights mix and the screen appears to be cyan, a blue-green color. When you hold the object in front of this cyan screen, you will see two shadows: one blue and one green. In one place the object blocks the light coming from the green bulb and therefore leaves a blue shadow; in another place it blocks the light from the blue bulb to make a green shadow. When you move the object close to the screen you will get a very dark (black) shadow, where the object blocks both lights.
When you turn off the green light, leaving the red and blue lights on, the screen will appear to be magenta, a mixture of red and blue. The shadows will be red and blue.
When you turn off the blue light, leaving the red and green lights on, the screen will appear to be yellow. The shadows will be red and green.


a like on this one please.

a red shadow can be formed 
the shadows are always black.