Most plastic sheets are birefringent due to the way they are produced. In the right circumstances, that can make a very colorful appearance when viewed in polarized light. As the difference in refraction index is normally rather small this works also for thicker sheets as no long coherence length is required for the effect to work. The effect should be strongest if you hold up the sheet to the clear sky with the sun behind you (the light coming from there is polarized horizontally). Sunglasses have the polarization vertically (to block exactly this light). As the birefringence turns the polarization the patch viewed through sheet becomes brighter. If you add in dispersion, you can get colored patches.

It's the same effect mineralogists use with polarizing microscopes.