I had no prior knowledge as I'm not big on cichlids, but google search "Red Frontosa" or "Frontosa Hypomelanistic". There's a lot of threads on it. But in short, it's said to be a selective/line bred trait, and it does seem frowned upon by the Frontosa keeping community.
But if you are saying it had black stripes, but turned brown, maybe it's stressed for one reason or another.