How Cones Sense Color
The cones are wired in such a way that they compare their activity to that of surrounding cones. If there were only one type of cone, this would only allow detection of contrast by measuring intensity difference.
People with normal color vision have 3 varieties of cones (we can call them blue, green and red, although it is more scientific to refer to them as short, medium and long wavelength). The activity of each cone is compared to an average of the activity of several hundred surrounding cones.
It is this differential response, not the absolute response level of a cone, that is transmitted forward to the visual processing centers. The retina is therefore a color contrast detector.
More detail
Each cone connects to 2 bipolar cells. One is more active if the cone in the center is less active than the surround (center OFF) and the other is more active if the center is more active than the average in the surround (center ON).
This average of surrounding cones is mainly derived from green and red cones, since blue cones comprise only about 5% of the cones. It is transmitted by horizontal cell input.
This arrangement creates 2 major color contrast channels: red-green (contrast between either a red or green cone and the average of both of them in the hundreds of surrounding cones) and blue-yellow (contrast between blue cones and the surround average formed by the red and green cones).