There are liquid solutions available.
usually 2 part solutions and a precipation reaction.(2 part solutions)
Gold, Silver, Nickel...
I can't find it right now
Also try dental supply houses for gold filling kits.
Search jewellery and engraving supply houses. If you want thin fills inside engraved lines and don't want to engrave the gold inlay itself, it should do what you want.
Enamels are also available to do the same thing in color, fill the indentatin and bake to cure.