Why can cones detect color but rods can't?
The spectral sensitivity of photoreceptors expressed is the key to color vision. See figure below for the sensitivity of three-types of cone cells (S, M, L) and rod cell (R, dashed line).


From this figure, one can say rod cells provide information about the "blue-greenness" of vision, however, despite their spectral sensitivity, it seems that in human vision rod cells do not contribute to color vision, because they are highly sensitive to intensity, and thus they are mostly saturated in their response (does not induce firing of downstream bipolar cells) during normal daylight conditions. Rod cells specialize for night vision (scotopic conditions) which is crucial for survival, and under this condition the cone cells are pretty much useless.