Students "oooh and ahhh" over color addition; they never believe that red and green make yellow until they see it and some of them not even then. I have a few favorite color addition demos: "Colored Shadows" from The Exploratorium Snackbook; a Mysterious Glowing Ball from Educational Innovations; and this year a color changing egg. I found this at a local toy store and bought it for my son for Easter. It cycles through colors and I ask my students how it is able to create six different colors. Most students come to realize that there are only three LEDs in the egg; sometimes only one is on and sometimes two.
Since my daughter broke the first one we bought (she wanted to play with her brother's toy—she replaced it with coins from her piggy bank) I plan to take it apart so I can show my students the inner LED lights without the translucent white egg exterior. At less than $10 from a variety of sources (Amazon, etc.) it is a cheap and easy demonstration of color addition. Below is a video I took of it cycling through colors although it is better to see it in person.