The photographic image is degraded by flare. Flare is stray light that reverberates around inside the lens and inside the camera. The camera lens consists of multiple polished glass or plastic surfaces and maybe a mirror and if a digital, a cover glass over the image sensor. All these polished surfaces both transmit and reflect light. About 5% of the image forming rays are lost at each encounter with a polished surface due to reflection. Some of that light comingles with the image forming rays and bathes the film or image sensor. The result is flare and ghosting.

Modern lenses are coated with a thin film of fluoride or other minerals. In fact most lenses may have multiple coats on every polished surface. You can often see that the front element of camera lenses and binoculars has a yellow or perhaps rose tint. These are coated lenses.

A tip of the hat to Harold Dennis Taylor (English 1862-1942 Optician). Taylor obsreved in 1894 that old lenses of the same design passes more light than new ones. He deduced that the older lenses has accumulated a tarnish due to atmospheric pollution from coal burning and the like. Investigation showed that older lenses with this "bloom", passed 2% or more light than ones that were freshly polished. Taylor experimented with suphuretted hydrogen and other chemical to artificially age lenses and was granted patent 29,561/1904.

A modern optical system consist of multiple lens elements, some are dense flint some light crown and other mixes of glass. Such multiple elements mitigate aberrations that degrade the optical image. Also modern lenses are multi-coated. It is the thickness of the coat that does the trick to reduce reflections. Some lens have as many as 12 coats, one specific to one frequency of light.

Now the coat must be ¼ of the wavelength of the color it is to control. This is a super thin and coat and variations are the norm. It is coat thickness variations plus the makeup of the glass that slightly alters the final hue realized by a lens. No two coming off the line are exactly the same. That’s why the color cast of one lens will differ from another.