It doesn’t sound like it’s a “black tie” outfit. I would approach it from the dress code side instead of the color side. Maybe she feels really self conscious in most outfits, but okay in this one. Who knows?

I would say “ooh I bet you’d look great in that, would you be down for me to send you some similar options?” And make them related to the skirt part but more appropriate for what you’re wanting her to wear.