Forza Silicon's patent application US20150090863 "Stacked Photodiodes for Extended Dynamic Range and Low Light Color Discrimination" by Barmak Mansoorian, Daniel Van Blerkom proposes two (or more) PDs stacked one on top of the other to enhance the pixel DR, and, to some extent, its low light color separation:
"The novel image sensors of the invention, and methods of using them, are based on various stacked photodiode pixel designs. In one aspect, the stacked photodiode pixels of the invention provide a means of extending dynamic range. The designs comprise, in an exemplary embodiment, an upper and a lower photodiode. In the event of a high photon flux wherein the upper photodiode becomes saturated, the attenuated light received by the lower photodiode may be used to determine the photon flux incident upon the pixel, extending the dynamic range of the system beyond what is capable with a single photodiode.
In another aspect, the stacked image sensors and associated methods of the invention provide a system for color discrimination in low light conditions. In combination with the signal received by the upper photodiode, the attenuated light signal received by the lower photodiode may be used to determine color, based on the predicted attenuation profile of the device, which is wavelength dependent."