Duo-tone antiquing is something that I just happened on when I decided to renovate some earrings that I wasn't to pleased with.
Some of the earrings were antiqued with just black and that went fairly well. Some I antiqued using other colours and they really didn't turn my crank, so I antiqued them again with black and found that there was a subtle difference between them and the earrings that were just done with black.
The earrings were made using my Teapot Tempest texture sheet.
If you just let the paint dry between layers but don't rub most of it off with each layer you get the kind of antiquing that you see in the right hand pictures of the Buddhas and the flowers.
I will play around with this technique more in the future.