The reason you're obsessing about your appearance right now may be more related to how you're feeling emotionally than it does to how you actually look. As you said, your hair isn't perfect, but it's livable.
This worry about appearances is likely due to the re-surgence of your feelings of competition with your in-laws. Since their lives look so perfect to you, you might be worried about anything about yourself that you feel isn't perfect. You can't really control your financial situation and the problems with your intimacy, so it's "easier" in a way to worry about outward appearances, because it feels like you do have some control over them.
But in the long run, spending more money when it isn't absolutely necessary will just make you feel worse, as you'll have added to your financial problems for something that will grow out (or need to be re-touched) in a month or two anyway.
We talked yesterday about finding self-esteem and self-worth in things that have real value--like caring for your loved ones. Making the commitment to wait until you actually need it re-done will let you be proud of yourself for putting your and your family's financial security first...and that will make you feel a lot better inside than an adjustment to your hair color ever would.