The primary colors are red, yellow, and blue for pigments but red, green, and blue in the light mixing system. These two sets of three are referred to as primary because they are the essential colors required for color perception. In the case of pigments, all colors are mixed from the primaries but the primaries can not be mixed from anything else. In the light mixing system, the primaries represent the peak sensitivity of the photoreceptors in our eyes. This is why all monitors mix RGB (red, green, and blue) to create the spectrum of colors you see in images and graphics on the web or television.
Color carries information that effects our emotions as well as our sense of space. Much of what we feel when we look at color relates to practical issues associated with human evolution. From both an emotional and environmental perspective blue is cool and red and yellow are warm or even hot. It's difficult to escape these color associations. When viewing open space, the color yellow is most quickly filtered out by the atmosphere over distance. Blues are least affected over distance and so far off mountains look blue to us and we rarely see yellow on their slopes or within the shapes of their silhouettes. In pictorial space it is therefore quite difficult to make yellow recede and blue advance unless this is done with some skill. Our sense of space and distance are shaped by our color sense.
It is important to understand the idea of complimentary colors when creating images and graphics. Complimentary colors are found across from each other on the color wheel, a useful tool for understanding color first created by Isaac Newton. Complimentary colors clash vibrantly. If I want something to look very red I can increase "redness" by placing something green close to it. I can intensify color by bordering color shapes with lines including the color compliment. For example, If I want my blue to look more blue I might encircle it with a line including enough orange to make the edge more vibrant. To learn more about pigment look up the "color wheel", a useful guide to understanding how pigment colors relate to each other.