Red, blue, and green are called a __ __ __ __ __ __ __ primary colors because when they are added together, they form all of the colors of the spectrum.

Additive

Additive primary colors are blue red and green

Red, blue and green are called Additive primary colors which is known as colours of spectrum.

Explanation:

A set of colors that are joined to produce some useful colors are known as Primary colors. Red, blue and green colors will not be formed by adding or combining other colors. Blue, green and red are called primary colors and these colors can be combined to generate various colors.  

When the colors yellow and cyan are added it generates a green shade. When the same yellow color is added with magenta it will generate shades of red. Cyan, Magenta, yellow and Black be used to generate color space.