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.
But simply taking that "frontend" BaseColor/Metallic parametrization out of BSDF and have it as a separate entity either at runtime or texture build time solves most of the issues. DiffuseAlbedo/SpecularF0 is blendable, more fundamental and more expressive.
... I just find it a bit ironic that something called "Metallic" can't represent real-world metallic cases well. The fundamental idea - there is different between "Frontend" BSDF and a "Backend" BSDF especially for real-time graphics.
in addition to that "Frontend/Backend" parametrization difference is crucial even for the most basic Metallic PBR as described here:
https://twitter.com/and_coder/status/1471079259300978690
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