Let the kids have fun with colors. With colors they can have fun while learning. Let them know that they produce one color when combining two or more colors. They will surely wonder and their curiosity will surely make them do it.
With the primary color yellow, red and blue along with white and black you can complete the rainbow color. The only colors that you cannot produce when mixing two different colors are yellow, red and blue, the primary colors.
Orange, violet and green are the categorized as the secondary colors. Each of these secondary colors is mixture of the two of the primary color. Let the kids combine red and yellow to have orange. For violet is blue and red. Yellow and blue produces green.
Red-violet, red-orange, yellow-orange, yellow-green, blue-violet and blue-green are tertiary colors. This is the product of a primary and a secondary color. To produce red-violet combines red and violet. When red and orange is mix it produces red-orange. Yellow and orange when mix it produces yellow-orange. For yellow-green from the name itself from the color yellow and green. Blue-violet is the combination of blue and violet. And for the blue-green I guess you already know what combination of color it from, if your guess is blue and green your guess is right.
For white, black and gray, they are known as neutral colors and achromatic. They are hues or not a true color.
Mixing different colors is a great activity for kids. They will think it like magic after producing different and new color. To get more about this mixing of colors see the color wheel.