Primary colors are green, red and blue. If you were to intersect three circles -- one red, one green and one blue --- then where the three colors overlap or intersect produces the color white. The simple way to see this at work is if you were to go backstage at a musical production and looked up into the wings on the light booms. You will see individual lights in green, red and blue. When the lighting director combines those lights together on a particular spot or through color gels, you will get the white spotlight.
Red, green and blue are the primary colors once again which when combined, create white where the three colors intersect. But where only two colors intersect or meld, that creates the additive color. Red and green together produce the secondary color yellow. Mix green and blue and it produces the secondary color, cyan. Mix red and blue and you get the secondary color magenta. If you work with printing machines and regularly change out color cartridges, the secondary colors plus a black cartridge are the four colors used. In the printing industry, CMYK represents cyan, magenta, yellow and black. In the subtractive group, cyan, magenta and yellow are now the primary colors. Where all three secondary colors intersect and meld together, black is produced. White is the absence of color.
RGB represents the red, green and blue of the additive group. When working on the computer in a photo color program, you would be working in RGB mode, or pixel mode, to get the correct colors by resolution. The more pixels, the better the resolution. Sometimes when you blow up the picture on a computer screen, you can find the odd ball white pixel which means there is an equal amount of red, green and blue in the pixel itself. Best way to cure this is to use the software's tool to grab the right color from another adjacent pixel, copy it, and then paste it into the white pixel. Once all pixels have been adjusted then convert to CMYK for ready-to-print status.
Your eye has rod and cones in the retina which processes light and, subsequently, color variations. In bright light, your eye can process color variations better than in low light and dark situations. In cases of subtle eye damage from age or injury, colors can be processed differently within an individual eye. In working with colors on a computer, each color available carries a specific number which allows you to reproduce a color within a picture, regardless of how your eye sees it.