If I'm understanding you correctly, that's not quite how it works. Film is made of a stack of layers as shown here.
Speaking broadly, we can discuss only three layers, the red, green, and blue sensitive emulsions.
When light passes through them in the "correct" direction, the blue sensitive emulsion absorbs the highest wavelengths of light, while the lower wavelengths pass through. The same applies to the green and red layers.
These subsequent layers are not desensitized to higher wavelengths though, the higher wavelengths have been removed by the previous layers. In the case of blue light, an extra yellow filter is included to absorb the excess high-energy light.
When you shoot through the film back to front, the highly sensitive red layer will react to all wavelengths of light. In turn, the green layer will react to green and blue light, and once it passes through the filters, the remaining blue light will expose the blue layer.
The total amount of light is reduced as it passes through the layers as well, so the green layer gets half the light the red layer received, and the blue layer about half of that.
This leaves the green and blue layers underexposed. By shooting at a higher iso, you overexpose the red layer—usually to the point where it maxes out—allowing you to increase the relative exposure of the remaining layers resulting in different color effects.
This is merely a rough emulation of redscale, but it demonstrates the distinctive "yellow shifting" as the ISO increases (each panel has been push processed the respective number of stops).