Well, there are myriad reasons why she could've said it.
* She truly thought it wasn't a good look - whether the color, cut, some combo, or something else
* She could be one of those people who feel she needs to criticize everything, without ever mentioning positives
* She could be jealous of you getting a chance to model, and this was a way of lashing out
And I'm sure there are plenty more options if I wanted to expend the brain power to think them up. But, really, I don't think it matters in the grand scheme of things. Here's my advice, for whatever advice from an old dude is worth to you...
1. If you really want to know why she said it, ask her.
Does she have a habit of saying mean things? Or is this a one-off? If this is a one-off, it's quite fair for you to tell her that her comments hurt you, and then give her the opportunity to respond. Remember that it's instinct for most people to try to defend themselves in this sort of situation, rather than apologize. So be prepared for her to try and justify her comments, stay calm, listen to what she has to say, and then work through it from there.
On the other hand, if she regularly says mean things, to you and others, you might want to consider why she's worthy of being your best friend. If it's been a habit of hers up until now, it's fair to assume that the habit will continue as time goes on. If she has other qualities that you like (which seems likely), you at least need to arm yourself mentally to be prepared for this sort of thing in the future. That brings me to...
2. This is the sort of advice that's much easier said than done. But... ignore her critique(s). When you boil it all down, I guess, why do you really care that she thought the red dress looked terrible on you? More importantly, what did you think of it? What did your cousin (who you were modeling for) think of it?
A great, but difficult, life skill is to take in criticism from somebody (anybody), process it rationally, and then decide if it's something worth acting upon or not. And if it's not, discarding the criticism with a short, "Thanks for your feedback" type of response is the best course of action.
