Tobbe I do not think the color should symbolize anything.
Instead, a selection according to preference from the user.
In the real world, I agree... especially for unwilling wearers who can benefit from feeling they still have some control over their lives. ...but, in fiction, it can be a useful thing to discuss.
That depends. Why do they use different colors?
If they only announce innocence and virtuousness, then she would continue to wear what she wore while married unless she cheated the belt while unmarried.
If they announce marital status and the setting doesn't allow divorce, maybe widows wear black because it's also the color of mourning.
Maybe divorced women's belts get two-color waistbands and the part where the colors meet is hidden behind the steel. White on one edge for being unmarried, pink or red on the other edge for having sexual experience.
