Colorwise, I think I would favor just trying a basic mix of some burnt umber at first, and then see where you are with a test strip. The nice thing about gum is that as long as your paper is sized well, you can back off on the amount of dichromate in the mix and increase the contrast of the gum layer. What this will allow you to do is to pretty precisely determine where the gum drops off completely so you don't sully the white of the dress. This will, of course, require some test strips. Because you are trying for subtlety, I expect, you probably don't want a super heavy pigment load in the first place. I am just completely pulling this out of thin air, but what I would try first is something like 10ml gum, .5g burnt umber, and 5ml of saturated am-dichromate. Do a test strip going from about 50% of your pd exposure time up to 100% in 10% increments, and make sure the various exposures on the strip covers both the flesh tones and the dress. Check it out, and see where you are. As they always say in the cookbooks: then season to taste.