Well, that's easy from one point of view, and hard from another... :-)
A certain "hue", "saturation" and "brightness" can be measured, but only if you give the measurement some certain starting points. Then you have to decide by what metric the distances are to be measured in, which is not easy as humans are a lot more sensitive to hue-change in some hues, and more sensitive to other hue changes in darker lighting conditions or other lighting temperatures. same goes for brightness changes in different hues. That's also why it CAN'T be objectively measured, the metrics are (and cannot be!) the same as the ones used for measuring reality. You have to stay within the target output referred space, with the set whitepoint of that media. Those will never be the same as "reality". Well, they CAN be, but that's only the case in very specific situations, not likely to correspond to any photograph that you can take in an actual, physical situation...
"Approximate difference" means that when something that looks a certain green in a certain light and another thing looks a certain red in the same light, they will appear to have the same inter-relation to each other when reproduced in/on another media. That if they seemed to have the same saturation and brightness in reality, that that impression is preserved in the output referred media. How that is done, or "measured" is up to you. Others have spent years of their lives doing this, and there are still several thousand man-hours per day spent around the world on this in research centers, software development companies, print-shops, pre-press development departments and so on doing exactly this.... When you have a definite answer, give me a call. I'll be wanting to get in on that.