Yellow is a primary color
what is an other primary color? 
Red is a primary color in different models
what kind of models? 
The RGB color model - it is the R
thats really cool! are there any other different models?
Color space
A color space is a specific organization of colors. In combination with physical device profiling, it allows for reproducible representations of color, in both analog and digital representations. A color space may be arbitrary, with particular colors assigned to a set of physical color swatches and corresponding assigned color names or numbers such as with the Pantone collection, or structured mathematically as with the NCS System, Adobe RGB and sRGB. A "color model" is an abstract mathematical model describing the way colors can be represented as tuples
in principle be used, but with red, green and blue the largest portion of the human color space can be captured. Unfortunately there is no exact consensus as to what loci in the chromaticity diagram the red, green, and blue colors should have, so the same RGB values can give rise to slightly different colors on different screens.
Additive and subtractive color models CMYK color model.
It is possible to achieve a large range of colors seen by humans by combining cyan, magenta, and yellow transparent dyes/inks