An excellent example of the effect of polarized light on clear plastic. Linear-polarized light is coming out of the polarizer (even though it is a circular polarizer - this is perfectly normal and doesn't indicate a fault), having been reflected by the white back, then passes through the clear cover where the colours are created.
You don't lose much, if anything, by using a circular polarizer instead of a linear. A linear may fool autofocus systems (not all of them) and exposure meters.
Maybe there should be a sticky, or an article on polarizing filers and their properties.