Hi,
Despite my reference images being black and white (and with small resolution too) I managed to complete the challenge, I went a step further and began to add the hairs, eyebrows etc …
The beard has 2 particles systems in it with 2 colors (brown-ish and blond-ish). I think it looks quite good. 
And if you wanted to see my model without hairs, here it comes with a comparison on the contribution of each maps:
The color changes are quite subtle except for the mouth.
The temple area is less shiny with the gloss map ON (right picture) I wonder if Fresnel is still applied with a gloss map plugged into a Principled Shader’s Roughness input
. But at least the nose and mouth part are shinier which is good 
I am happy with my model, even if it doesn’t look exactly like the reference.
Cheers !