I know that silicon and even black plastic bags are transparent to infrared light. What about light-colored silicone polymers (take white or green colors)? Are they transparent to infrared light too? And why?
Silicone polymers are transparent. They do absorb some IR light: there is an infrared spectrum here, or Google for more examples, but they are only opaque if some filler has been added.
So whether or not any particular silicone polymers will transmit IR depends on what filler has been used in it. I think fillers tend to be aluminosilicates or metal oxides. Aluminosilicates transmit some IR light, but note that because the filler is a powder there will be a lot of scattering at particle boundaries so IR light is probably scattered by reflection just like visible light.