[geon]

The photo cells in the eyes aren't just sensitive to red green and blue, but a range of reds, greens and blues.

As I understand it, people with mild red-green color blindness have trouble seeing color because the sensitivity range of their red/green photo sensitive cells overlap too much.

The filter in the glasses cuts out a portion of the red/green spectra, so that the overlapping is reduced. That makes the red photo sensitive cells receive only the deeply red light, and vice versa, creating better color channel separation.