M.D.
L.,
I'm so sorry what you're going through. You are not being overprotective. Don't let anyone convince you otherwise. I frequently have similar problems concerning my 12 year old daughter and what is considered acceptable when she is with her father, who now lives with his girlfriend. Trying to make the "opposing" party see reason is pointless in cases like ours. You can't win that one. Let it go. It's not your ex you have to convince, it's your daughter. Every kid is unique, so I'm not saying which of these, if any, might do you some good. For instance, there is a lot of speculation concerning the long term effects of makeup now. It seems that there is at least a theory about the various chemicals involved in the cosmetics. Supposedly chemicals get absorbed through the skin, which can have all kinds of disastrous consequences, like cancers. I'm not real familiar with exactly the details on that, and I wouldn't know how to begin to try to research it, but it might be worth looking into. And of course continue to reassure her of how beautiful she is. This is a very sensitive age and with the split family and all she really has quite a lot to have to deal with in addition to the other things that are normal for the age.
Another thing is that you can use this to teach her about different standards of appropriateness. Just because it's ok at dad's doesn't mean it's ok everywhere. Just like there are clothes that are suitable for school that could never be worn to church, and clothes that can be worn at home but only inside. Just like you can dress for McDonald's and be fine, as long as that's the kind of place you were at. But if you were dressed for McDonald's and wanted to be eating somewhere better they might not even let you in. Real life as an adult is about understanding what the standards are for each of the environments that we encounter, and respecting those boundaries. A big one that she should already be familiar with is that you only dress for Halloween on Halloween, and for Halloween parties, but those costumes are just not acceptable at other times even when there's really nothing objectionable about the costume at all. So she needs to be able to relate this to the fact that the rules at her dad's are different than the rules at home, and she needs to respect the rules of each of you when she is with you. I know it sucks, not to be able to control her behavior when she is away from you...but this is part of the consequences we have to live with. And, honestly, even people who stay married forever still someday have to face the fact that they can't control their child's behavior when the child is away from them. You can't control what she does at school, either. And you can't control what she does if she goes to visit friends or attends slumber parties. This is the hard part of mothering, having to recognize that you and your desires are no longer the end-all and be-all to the child's existence like they were when she was just a baby, all helpless and dependent on you. Or when she was a toddler and still believed in the omnipotence of her parents. Or when she was a sweet little gullible kid, still believing in magic and Santa Clause. As hard as it was giving birth, as hard as it was raising her this far, I hate to tell you but it only gets harder from here.
Sorry, rambled way to much, I'm sure. The point is, she has to want to do the right thing, regardless of whether you're there to make her or not. And she has to respect your house rules when she is in your house. Deep down inside, she needs to know that you care about her enough to do the right thing even though she doesn't like it. Boundaries. Limitations. They suck, but they're for her own good, and it may take years but she'll thank you for it one day:)