The “index of refraction” is a term in optics referring to how much a lens material bends (refracts) light going through it. For example, air does not bend light as much as water does -something readily apparent to us when we see a pole sticking up from the surface of a pond. It is not really bent!
The natural human lens of the eye does not have a single refractive index. Rather, it grows like an onion: from a very dense center to successively softer outer layers kind of like a tree with its rings. So there is a series of progressively more dense/higher index bending of light as a ray passes through the human lens. This arrangement (called a gradient index) damps down reflections and improves the optical quality of an image.
Lens implants or IOLs (Intraocular lens) do not have much of this quality so they tend to reflect light and filter out some colors due to the progressive discoloration of the human eye with age.
There is no IOL today that can match the optical quality of the natural lens we are born with. But new IOL technology continues to improve, so maybe someday…