It's what you sometimes see on a smartphone screen protector or on the edges of an LCD screen. The rainbow effect has been seen in CGC holders since our first-generation holder was introduced nearly 20 years ago. This is a result of the inner sleeve (which holds the book) contacting the hard-plastic outer shell; the two different plastics have different refractive indexes and that sometimes creates a rainbow effect when they come into contact. A minor rainbow effect has always been within our tolerance.
As such some of our CGC Graded Slabs may exhibit signs of "Newton Rings" this does not in anyway deviate from the overall grade or quality of the book graded.