The image that you see on your computer monitor is made up of a bunch of little dots (or possibly lines). If you have a color monitor, there are three different colors of dots: red, green, and blue. (That's why monitors are sometimes called "RGB".) To get different colors, the monitor mixes the dots.
It is possible to see the dots that make up the picture in a couple of different ways:
1) You can use a lens to magnify them. 2) You can make a lens to magnify them.
The second one is actually easier. All you need is a little bit of water. A drop of water on your screen will be convex, and so will work like a little lens!
Note: in what follows, you are asked to put a little bit of water on your screen. Do so very carefully! Put water only on the glass, and do not let it drip!
Take a bit of water on your fingertip, and touch your fingertip to the red square below; some water will stay behind.
Look closely at the water drop; you should see a magnified image of the dots or lines that make up the image on your screen. What colors do you see? Since red is one of the primary colors, you should see only red. Try the same thing with the green and blue squares; again, you should just see one color.
It's different with mixed colors. Put a dot on the white square. Your monitor makes white by mixing red, green and blue. So, you should see red, green and blue dots or lines! It might look like a little rainbow. Move your head back and forth in front of the water dot; you should see the pattern move.
OK, now for the big test: what colors are used to make up yellow? Put a dot of water on the yellow square, and find out! The result might surprise you: yellow is made up of red and green!
And how about purple? Well, we will leave that one to you.
Now, when you worked with crayons in kindergarten, you were told that yellow plus blue makes green. And now we are telling you that green is a pure color, and that red plus green makes yellow. Who is lying - the Little Shop of Physics, or your kindergarten teacher?
The comforting answer is: neither. We are both right. Your screen gives off light; you can see this in a dark room. (More on this later.) So, on your screen, you are mixing colors of light; this is called additive color mixing. Crayons don't give off light; they absorb it. So mixing colors of crayons is subtractive color mixing. The results are different.