One of the color wheels is the quadrant, which is divided into four parts. In the quadrant four primary colors are placed according to four coordinate axes, pointing to the four cardinal points with 90 degrees. The color wheel from top to bottom, clockwise according to the primary colors: red, yellow (or orange), green, blue.
A color wheel is actually consists of three primary colors, three primary colors can be mixed for other colors.
When mixing paints the red, blue and yellow are considered basic color, because blue and yellow paint can be mixed for the color green. As a point of view on the color wheel quadrant the yellow and green color can also be considered as basic colors. In the case of computing or television sets, the three primary colors are red, green and blue, or RGB. So both green and yellow can be considered as primary colors.