What determines the colour of viewed light that has been linearly then circularly polarised and viewed through a linear polarising filter? I have run a little science experiment using a white light table with a linear polarising filter atop it and gotten students to stick sellotape on acetate to act as a circular polariser so that when viewed through 3D glasses (linear polarisers) it reveals different colours based on the angle and rotation from which they are viewed as well as layering of the sellotape. Pretty much as shown at the beginning of this video.
I have a relatively good grasp of scientific concepts; my current understanding is that elliptical/circular polarisation occurs when light is refracted through an anisotropic medium, which in this case is the sellotape (and acetate to some extent) and this slows down one component of the EM waves. 
But what is it that determines the colour you see when you view the circularly polarised light through another linear polarising filter (3D glasses)?
Thank you.
 A: Simplified, it's interference patterns across different wavelengths based on the thickness of the material.
If you used monochromatic light, you would likely see an interference pattern that was related to the thickness of the material.  The index of refraction differs based on the polarization orientation.  At some thicknesses the phase of each will match on exit (same as if it had a uniform index of refraction) and at other thicknesses the two directions will be out of phase.
But because the indices of refraction differ with wavelength, the thickness that allows red light to be seen through the polarizers may not allow green light to be seen through them. 
Using some optical filters should show a much simpler pattern on the materials.  

so it is simpler to see this as a problem of diffraction as the chains
  of hydrocarbons in the sellotape will act as a diffraction grid and
  depending on the angle of viewing I will get either a constructive or
  destructive fringe

The interference isn't from the chains directly.  Instead the chains change the index of refraction based on the orientation of the light's electric field.  This turns the piece into a waveplate. 
Different thicknesses mean it might act like a half-wave plate for some wavelengths (reversing the direction of the circular polarization and allowing the light to be visible), and it might act like a full-wave plate for others (leaving it alone for the second filter to absorb the wave.  
