I have been reading Mahnke's 'Color, Environment & Human Response", in which I've found a few references to how certain colors effect the eye. The focal point for red, for example, is behind the retina, so it requires the eye to refocus. I wonder if there is any source for this information for other colors - how the effect the eye.

Each color of the rainbow actually has its own focal length which depends upon the wavelength of the light. Red is a longer wavelength and it has a longer focal length. Blue is a shorter wavelength and has a shorter focal length, green is somewhere in between. A prism separates the colors of white light into the colors of the rainbow based on wavelength. So the familiar colors of the rainbow --red, orange, yellow, green, blue, indigo and violet are sorted from longest to shortest wavelength. Through a given lens, they would all focus at slightly different distances. Fortunately, the eye has evolved to manage this problem very neatly, so unless we are trying to focus very bright, specific hues, we usually don't notice it. Artists sometimes use this effect by placing bright red and green or orange and blue close together, in stripes for instance, which results in almost a visual vibration as the eye and brain work to reconcile the image. There are many, many books that discuss the physics and physiology of color vision. One book I happen to have on my desk is Real World Color Management by Fraser, Murphy, and Bunting. Although it is a fairly technical book, the first few chapters do a nice job of explaining how color vision works. Actually, Wikipedia is a good place to start (search for "color vision").

in regard to the gray/black color achieved by mixing the 3 primary colors... do you achieve the same exact gray/black when mixing equal parts of the complementary colors (regardless of the particular complementary colors being mixed)?

This is a great question which would require a book, or at least a chapter to answer completely. The simplest answer is yes, the same gray or black can be made with two different sets of complementary primaries. The most important factor is the room lighting when this color comparison is made. Under a given light source it is possible to make two grays that match perfectly even though one is made with blue and orange and another is made with red and green. But, if the lighting is changed then the two grays may not match. This property is called metamerism and understanding it is very important to good color matching. Metamerism doesn't just apply to black and gray, it can occur with any color pair where different pigments are used to match the color. They will match under one light source but not necessarily under another one.