The lyrics of Rouge no Dengon are really good. It's about a woman who's on a train, leaving town to get away from her cheating husband. She left a "message in rouge" for him to find in the bathroom, that she won't be back until he changes his ways. She's actually going to his mother's house, and she'll have his mother call him to scold him in the morning.

If you're familiar with the song as the opening theme to Kiki's Delivery Service, it really really fits Kiki's character as she listens to the song while leaving home for the first time. You might get the feeling that the themes of the song are things relates to herself: independence, anxiety, maturity, and female solidarity. And of course, if you watch the movie, those are all things she experiences in her new town.

Basically it's a REALLY REALLY good song and absolutely perfect choice for the movie it's in. I also love the singer's other Ghibli movie song you mention, Contrails/Hikoukigumo.