Why do German Rieslings come in different-colored bottles?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
Could you tell me if Riesling in a blue bottle means that it is cheap, sweet Riesling? What do the different colors and shapes mean in German Riesling bottles?
—Cecilia, Buenos Aires, Argentina
Dear Cecilia,
Traditionally, German Rieslings were bottled in either green or brown glass. Brown glass meant the wine was from the Rheingau region, green from the Mosel. About 30 years ago, some producers switched to blue bottles to distinguish themselves, the most famous of which was the German Riesling line Blue Nun. I’ve seen other wines—not just Rieslings—bottled in blue glass since.
While I’ve seen a fair amount of value-priced, softly sweet Rieslings in blue bottles, the color of the glass alone doesn’t really tell you everything you need to know about the wine’s profile or price.
—Dr. Vinny