We have worn a matching harness over outfits. This is the one she has that matches best Gracie has a couple harnesses with the d ring close to the neck area that we use when we want the harness under. This is one of them, although for her angel princess costume we put the harness over the dress and under the wings. We have never used a collar on her. I don't think she would react well at all if we tried one. Her breeder always used a harness too.
@LariP, that is definitely a wardrobe fit for a princess! she's such a darling!! that first picture harness blends in really well with her clothing. The 2nd pic, i cannot even tell which part of the outfit is the harness. I don't think the boys will do well w/collars either and i do prefer the harness route.