The three kinds of cones differ in their response to incoming light and similarly to the absorption spectrum of objects, the retinal cones have a response spectrum describing their intensity of neural activity triggered by a cone as a function of the wavelength of incoming light. The intensity of neural activity resulting from light entering the eye can thus be simply read off these curves. For a mix of wavelengths we can integrate to find the total stimulation intensity per cone. The visual cortex performs the same integration of the signals and the final experience of colour is based on the signals of all three cones. Of course the specific response curves vary from person to person. Examples of a retinal stimulation by different light sources can be found here. We can now follow light emitted from a source, interacting with objects and entering the eye to ultimately trigger a perception of colour.