In the absence of color or when no color is showing, the result is black. In additive mixing of colors, the primary colors are combined in different ratios and the human eye will see the sum of the primary colors’ average brightness.  If all three primary colors are showing in the same proportion, the result is white.  When red and green are combined in equal proportion, the result is yellow.  When red and blue are combined in equal proportion, the result is magenta.