Blender Guru always taught me to keep away from changing the Specular channel of the Principled BSDF node (‘unless you’re a pro’) and to use its Roughness channel instead.
But how does this translate to the Glossy BSDF node? Am I supposed to leave its Color channel alone and to just use its Roughness channel?
for realistic result always leave the glossy color to white (unless it’s metal like copper or gold)
The reflection (glossy) part of a dielectric is “always” white. The fresnel node is used to control how much of this whiteness is actually used. Look at a colored plastic piece you have at home. At glancing angles your ceiling light will reflect at its original color. For a colored metal like brass or bronze, the reflected lightsource will be tinted.
Principled will handle all this automatically, as well as reduce the fresnel based on roughness. Principled also allows you to tint the facing reflections towards the base color. It’s artistic and not “realistic” per se, but you can use it if you have the surface “covered in tiny colored hairs” that would influence the coloring due to transmission and absorption of the hair itself, without actually setting it up physically correct.
I use glossy a lot for metals, particularly the anisotropic shader, since Principled doesn’t support Bechman distribution which I find really useful for smooth glossy with high anisotropic highlight stretching. I also use glossy when setting up any kind of glass or transmissive materials, as manual control allows me separate control over roughnesses (and color).
I don´t really use principled so I´ve got a question. Isn´t it really limiting if you can not control the fresnel falloff yourself? Or can you control it in some way?
I ussually use a layer weight node coupled with an rgb-curve node to blend a diffuse and a glossy node. This lets me start with “realistic” falloffs and then give it “artistic” tweaks.
How do you influence the falloff with principled when it is all done automatically?
Specular value controls the falloff. Default 0.5 gives about 1.45 or 1.5 (can’t remember which), which is close enough for most of the dielectrics. Use higher for more brilliant surfaces (glass or high IOR epoxy coat) and less for less reflective materials (like water). It’s a direct relation to IOR, but in more artistic friendly terms (texture and value wise). Manual has a formula that shows the relation, but a 0-1 value is much more convenient to preview and texture.
The problem of using layer weight, is that you can’t create realistic transmissive materials such as glass with it as the IOR needs to be inverted for backfacing faces (when you have an actual volume). For thin glass or translucent materials like leaves, layer weight would be a blessing as you don’t have to consider the backfacing faces.
For dielectrics, the facing curve should be 4% at facing angles and 100% at glazing angles.
A confusing aspect is that layer weight/fresnel, which uses 0-1 values, is not scaled the same way as the principled specular value. Not sure if it’s supposed to be backward compatible or just forgotten.
So… are the Specular channel of the Principled BSDF node and the Color channel of the Glossy BSDF node the exact same thing?
Sorry, it’s quite hard for me to get since I am used to only work with Principled BSDF… 
Ok, i guess that is enough for a lot of things but a curve is still a lot more flexible. I mean with a slider your choice is rather limited. With a curve you can insert as many points as you want to and control every aspect of the reflection.
Kind of, as long as the color is white (for dielectrics). But not really, since principled reflection automatically scales with roughness which is something you have to do manually when using glossy.
As far as dielectrics goes, glossy = always white (0.8 is safer), and you have no control over it. Which complies with the metallic workflow.