Thickness ...
Optical anti-reflection coatings were still a bit of a challenge in the early '50s, but by the end of the decade they had evolved significantly. The thickness of the layer determines the dominant color seen in reflections. The thickness is determined by the exact chemical makeup of the coating. As coating tech evolved, new coating varied in thickness. The color you see is not a tinting, it is the result of interference between the light reflecting from the two surfaces. Google "Newton's Rings" for the explanation of the physics involved there.
The magic of such coatings is that while a single-layer coated lens has twice as many reflections, one from the air-coating boundary and one from the coating-glass boundary, the total brightness of those two reflections is less that of a single air-glass boundary. This happens when the coating has a refractive index between that of air and glass.
Someone, I believe it was a Schneider-Kreutznach, figured out that the best results occurred if the refractive index of the coating was exactly half way between air and the particular glass being coated. Since part of the magic of optical lens systems is that they use a mix of glasses of differing refractive index, coatings aren't a one-size-fits-all thing. They developed a variety of coatings, each one optimized for a different glass. This idea quickly spread throughout the better optical manufacturers.
