I have never heard of mixing primer with paint. Primer is ordinarily used before the first coat of colored paint.
The purpose of adding white is the so-called "scale effect." When objects are viewed from a long distance- especially dark objects, the intervening haze lightens the apparent color of the object. Now, this effect only occurs for very long distances. Personally I do not do it to anything other than ships of 350th scale or smaller. One might argue that 1:144 aircraft should have it. I have worked with camouflage theory, and you usually do not get much lightening unless you are viewing something from 500 or 1000 feet away.
Normal distance for viewing some small object like a model might be 2 feet or so. So unless the object is about 1:250 scale you probably don't need to lighten colors.
However, there is another cause of lightening of dark colors. Older paints- 1960s and older- chalked readily when left in sunlight for extended periods. However, that chalking was primarily on upper surfaces. I personally feel the basic paint should not be lightened- only a fine airbrush coating of a lightened color over the top of fuselage, top of wing and tail surfaces to represent chalking. And modern paints- say 70s and later, are quite chalk-resistant.