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Real-world metal cross polarized examples. Nothing is ever pure, it's always a mixture. Left to right: actual photo, specular and diffuse only lit components. It is not uncommon to have differently colored diffuse and specular especially for metals. I hope Metallic PBR dies soon. 
by "Metallic" I mean BaseColor/Metallic to DiffuseColor/SpecF0 conversion that's not a big issue in offline rendering where you might work with pure materials and mix as many BSDFs as you want at nearly 0 cost. Here is an example of gradual gold deposition on top of blue plastic
However, in real-time application you only get to work with just a single "backend" BSDF parametrization. Need to collapse complex materials down to just one BSDF, like in #UnrealEngine, #glTF, #USDZ etc... Then you have a problem. You can't represent that mixture any longer.
