Hi. Excuse mi English.
The main difference using curves is that you always get full white reflectivity on glancing angles. Using a color, you don´t get 100% (Beacuse colors can´t be 255-255-255). But to be honest sometimes, the difference it´s barely noticeable. Here is an example i made to illustrate.
All metals have "tinted" reflection (specular reflections are´t equal in all wavelengths), but the effect in some of them is barely noticeable. The most important ones are: Chromium, Platinum, Nickel, Titanium, Copper, Iron, Gold, Cobalt.
Aluminium and Silver are almost white.
Light wave have 2 different polarization, these Red and Blue curves in your graphic represent those polarization and the green one the "approximate" medium value, the one you should use.
To get the reflectivity by color, you need to introduce 3 (because you want to translate to a RGB sistem) diferent light wavelength in
http://refractiveindex.info/. Blue 0.440 / Green 0.510 / Red 0.650 (approximately, light it´s not that simple) and use ONLY the green curve of each one (you can see the example of gold in mi image).
Another different question is if all of this really pay off. Sometimes you can get almost the same result using a single color in a fraction of the time you need to spend tweaking the curves, and almost always you need to adjust by eye in both cases. It´s an excellent exercise to better understand light but you must decide if it is efficient.