About
Secondary colors are obtained by mixing equal parts of two primary colors. They are located between the primary color in the color wheel.
With 3 primary colors, you got three secondary colors.
By type of colorspace
Additive
In a additive colorspace (light, screen, monitor) such as rgb colors system, the secondary colors are <MATH> \begin{array}{rrrl} & yellow & = & red & + & green & \\ & magenta & = & red & + & blue & \\ & cyan & = & green & + & blue & \\ \end{array} </MATH>
blue + red is not purple as in the subtractive model but yellow
Subtractive
In a subtractive colorspace (ink, pigment, painting, printing) such as ryb colors system, the secondary colors are
<MATH> \begin{array}{rrrl} & & blue & + & red & = & purple \\ & & blue & + & yellow & = & green \\ & & red & + & yellow & = & orange \\ \end{array} </MATH>
blue + red is not yellow as in the additive model but purple.