I remember having seen a diagram showing more than three types of human photoreceptors.
It still had three base colors: RGB. Most photoreceptor types were displayed covering two out of three colors at once, so more of them were needed to distinguish individual RGB colors.
I've tried googling but haven't found any confirmation. All sources that I've managed to find support classic RGB cones and one type of rods.
Is my memory of the diagram false? Or are there recent discoveries which update the 3+1 human photoreceptor formula?