In response to hmartin430, my expertise is really in the use and application of CIE colour matching and display screen technologies, rather than the actual structure of the human eye.

My understanding too is that the rods are sensitive to low light (and saturate at higher light levels).

If we just consider the cones in the fovea, that comprises three types of cone: L,M,S (long, medium, short wavelength), which are very loosely red,green,blue. They are actually much broader bandwidth, with highly overlapping wavelength sensitivities than true RGB. The CIE colour matching functions (and resulting "chromaticity" coordinates) X,Y,Z are mathematically related to the L,M,S cone spectral sensitivities but are not quite the same thing (it's a long story...). The XYZ colour-matching functions are 'mathematically fudged' slightly such that the Y-coordinate represents luma (brightness) as well as (sort of) "green".

Grappling slightly for a consistent solution to all these things, I believe the answer is that in the fovea there is a highest density of M-cones, fewer L-cones, and fewer S-cones still. This means that our "luma" resolution is highest, red-green resolution is somewhat lower lower, and blue-yellow resolution the lowest. (In practice you need to match the luma (brightness) of the coloured test-stimuli to really demonstrate this effect, otherwise if "yellow" is much brighter than your "blue" it may be resolved in luma even if it isn't really resolved in chroma).

Again from a technological perspective, the Bayer colour filter array pattern used in the vast majority of electronic colour-camera sensors has twice as many green pixels as blue and red, which again maps to approaching human-eye properties to get the "best" visual image from finite technical resources.https://en.wikipedia.org/wiki/Bayer_filter