Several manufacturer uses rhodium plating above white gold because rhodium is harder etc. The reflectance spectra of white gold alloys (depending on other elements, like nickel, palladium etc.), different stainless steel qualities and platinium as well as rhodium are quite close each other. Everything is also related on the finnish of the surface, observaton light and also observer. The spectra are diffrent; however, the eye sees only 400...700 nm range that is almost similar in all the metals/alloys. It hard to say if it is possible physically to distinguish all these materials from each other.