Specifically - I’d like to reproduce the highlight effect on the raised portions of the frame. I guess the frame here has beige on black and I plan to use gold on black with the same effect.
My guess would be that they painted the whole thing black, then sprayed the grey/patina paint on and wiped it off with a rag.
That is, unless it was mass produced.
Does the frame you are staying with have physically raised texture like your example? The black parts are raised up, the lighter grey is in the recesses.
To achieve this effect with gold in the recesses, you can paint the whole area gold first, then use a sponge (not a brush) to just hit the raised areas with black.
I might be tempted to use a few extra steps and use gold leaf:
Paint the woke thing black. Let dry.
Mask off the textured portion with painter’s tape.
Apply adhesive to the area. Let dry.
Gently buff off the adhesive from the raised bits. Use a sponge that’s rough enough to rub off the glue, but gentle enough not to damage the black paint too much.
Apply the gold leaf, using a bristle brush to press it into the recessed areas. It should only stick to where the adhesive is.
Buff off any leafing material that has stuck to the raised areas.
Touch up black areas if necessary, then spray varnish.
Gold leaf is heaps better than gold paint. You need to work hard to make the surface smooth and non-porous and then use a special adhesive called size.