The traditional color wheel shows the three primary colors are red, yellow and blue. While this is true, it is misleading: red, yellow and blue are the recognizable primary colors used for pigment mixing, but the primary colors of light are red, green and blue. Computer monitors and televisions use the RGB spectrum to create every other color in their displays. For printing and pigment mixing, a more effective spectrum is used instead of red, yellow and blue primaries: cyan, magenta, yellow and black, or CMYK.
The reason computer monitors and television sets use the RGB spectrum is because they are light sources: they mix colors using additive mixing, or adding in more colors of light to create a new color. Pigment appears blue to our eyes when it absorbs every other color in the light spectrum and reflects only blue. When mixing colors of light, you add other colors to what is reflected; this is additive mixing and is used only in light mixing. In additive mixing, you can achieve yellow light by mixing blue and green.
When mixing pigments such as paints, crayons, markers or printing ink, subtractive printing is used: the more colors you add, the more light is absorbed and the darker the color becomes. This is why when children mix paints together, they end with a muddy brown color: the paint starts to absorb every color added, reflecting less and less, and getting closer to the complete absence of color, otherwise known as black.
You know from your experience of mixing paints that mixing blue and yellow results in green: this is not always the case for true blue, which is too dark: when yellow is added to true blue, the resulting color absorbs more light, reflecting less, and appearing darker to the eye. Using a lighter blue, like cyan, will yield green if it is in equal amounts. You can create two other secondary colors from mixing blue and yellow: 25 percent blue and 75 percent yellow results in yellow-green, while 75 percent blue and 25 percent yellow produces blue-green.