[The nature of this activity makes it difficult to photograph (at least for one with limited skills in that area, such as myself). Try it yourself for the best results.]
A little refresher on color and light...
--White light is composed of all the colors in the spectrum (red, orange, yelllow, green, blue, indigo, violet).
--An object appears red because it reflects the red light and absorbs all the other colors.
--An object appears white because it reflects all the colors in the spectrum, and absorbs none.
--An object appears black because it absorbs all the colors in the spectrum, and reflects none.
What you'll need...
--a red toy car
--a blue (or other non-red color) toy car
--a red light (this could be a laser pointer - be careful!, a filtered flashlight, a toy - mine came from a cereal box)
You'll need to do this demonstration in a darkened room. If you can't get your room dark enough, you can set it up inside a large cardboard box - but only a few people will be able to view it at a time.
To perform the demonstration...
Before darkening the room, show the students the two cars, so they can see that one appears red and the other blue.
Turn off the lights. Shine the red light on the red car.
The car will still appear red because it is reflecting the red light (it's not absorbing anything since you didn't shine any of the other spectrum colors on it, but that doesn't change what you see).
Now shine the red light on the blue car.
The car will appear black (or at least a dark shade - to get black you'd need a totally dark room and a pure red light).
Your light is only emitting red light. The blue car only reflects blue light. There is no blue light for the car to reflect. The red light is being absorbed by the car. The result, for our eyes, is black, or the absence of color.