Not sure if this pooch is actually old - he is called Smudgey - but he had that "old as the hills" look in his eyes as well as grey (or faded) fur. Reference photo by Jennifer Livick on PMP.
I worked this with a palette knife apart from the eyes and nostrils. I used black for the pupils and nostrils only.
The greys and other darks are mixed from Prussian Blue, Alizarin Crimson and - if it needed to be really dark - a touch of Rembrandt Transparent Oxide Brown.
This is a fab colour for darkening complementary greys but only a little is needed or it swamps the other colours, even Prussian Blue.
There is nothing else that swamps Prussian Blue in my experience, except perhaps for my own person: I finished this small painting and subsequently found little smudges and spots of it spread across a good deal of my clothing and even my body, including elbows, like an Ancient Briton experimenting with woad, perhaps. No idea how that happens.