Know what I noticed just now? Even though every single Rihanna album is different from the other, I noticed that there are 3 pairs that have similarities.
Music of the Sun
A Girl Like Me
Both are heavily influenced by Rihanna's bajan roots and give off the Island vibe she was known for in the early years of her career. Both are lighter albums and have a common Dancehall theme between them.
Good Girl Gone Bad
Rated R
Both are dark albums compared to the first two, though Rated R is obviously GGGB to the maximum level. GGGB is the bad girl version, while Rated R is the Gothic Chanel version. Both share the common dark themes for the eras, and the color black.
Loud
Talk That Talk
Although these two may seem drastically different on some parts, it can't be argued that songs like WFL and WHYB would have fit right in on Loud. The dance themes on both albums are incredibly similar, be it TTT takes it to the next level. In a way I see Loud and TTT as Yin/Yang; Loud being the lighter fun side, and TTT being the grittier dark version.
So that begs the question, what will the theme of the next era be?