Which paint and glue to use for a golden walnut?
For “medium- to large-scale production” (e.g. making golden walnuts as tree ornaments), we’ve successfully used both golden craft paint and spray paint. Especially the latter is a bit messy, but retains the surface structure very well.

Glue the halves together (wood glue or standard white craft glue will do), sandwiching a ribbon loop as hanger in between. For the spray option, protect the ribbon with masking tape, loop the hangers over a kind of rod, lining the ornaments up with a bit of distance in between. Spray, either outside or with a large cardboard box put on end to catch the excess paint. Let dry while still hanging on the holder. Remove tape, done.

For a hanger-less version, push a toothpick in the part of the shell where there’s a little natural gap, stick the other end in a piece of styrofoam or similar.