From the link ... "Many will insist that red-blue-yellow are the "primary colors" in an absolute sense because no combination of other (subtractive) colors will produce them. However, in additive color, red + green = yellow and no simple combination will yield green. In this sense red-blue-green may have the truer claim to being absolute primaries, since additive light works with color directly from its originating source (light). It should be noted that additive color is a result of the way the eye detects color, and is not a property of light. There is a vast difference between Yellow light, with a wavelength of approximately 580nm, and a mixture of red and green light. However, both stimulate our eyes in a similar manner, so we do not detect the difference." Sort of starts to explain it, great help! Thanks Richard