Why are there so many more rods than cones? They only explanation I could think of would be evolutionary: it might be more important to see very well in gray-scale under low light than in color under typical light levels from a fitness standpoint.
Short fact: Vitamin A deficiency can lead to poor vision and may eventually lead to blindness as the lack of vitamin A causes damage to the retina and causes the cornea to become very dry (link)
Is color blindness caused by having not enough cones with a particular spectral sensitivity?
Regarding question above: "People who are colour-blind, or dichromats, have only two cones and see perhaps 10,000 colours." (http://www.bbc.com/future/story/20150727-what-are-the-limits-of-human-vision)
Apparently, rods and cones are distributed unevenly across the retina. Cones are localized in the middle of the retina while rods are distributed outside. Our peripheral vision is almost exclusively picked up by rods! It's weird to think that our peripheral vision is mostly in greyscale...
Adding to the comment above, this means we can turn our head to use our peripheral vision to see the things in front us better at night!
It also has to do to how the cones respond to certain colors. For deuteranomaly (a red/green colorblindness), apparently the peak sensitivity is much closer for red/green cones than it should be.
As mentioned in a previous comment, cones are concentrated mostly in the middle of the retina, while rods are spread over the rest of the retina. Cones are for color, which means that a lot of our peripheral vision is actually grayscale; this might seem a little meaningless, but it actually helps a lot for increasing out field of view. Rods are also more sensitive to light than cones, which is part of why color is hard to see in dark places.
Adding to the discussion, there are researchers who claimed to have found individuals with the fourth cone, which they call the "tetrachromats". An article on this which you might find interesting: https://futurism.com/uk-woman-extra-cone-cell-her-eyes-can-see-more-colors.
So color blindness is rather common due to the little amount of cones?
The frequency of colorblindness is not so much due to the amount of cones as it is due to genes. According to the NIH, colorblindness affects about 8% of males and 0.5% of females of North European ancestry. The discrepancy between genders is due to the fact that colorblindness is carried on the X chromosome--as a result, "in females, a functional gene on only one of the X chromosomes is enough to compensate for the loss on the other".
On the note of tetrachromats — it's actually impossible to test if someone is a tetrachromat given a standard display screen (RGB), as the colors emitted from the screen will only have exist in a 3-dimensional basis; thus, trying to check if a person can view a 4th-dimensional color requires some other form of checking such that such a color can be represented.