i'll just assume you use photoshop
just experiment with different gradient maps/photo filters on different blending options and see what does what
to me, it kinda looks like a b/w filter on luminosity and/or a brightness/contrast adjustment, maybe also levels and curves
also when you're finished your piece, you can try making a layer and fill it with a solid color (like dark grey/blue), put it on exclusion and lower the opacity to <5%