As explained, it's a seasonal special version from a famous soda brand.
Mitsuya is a 130-year-old brand known for what we call "saidaa" in Japan, which is a sweetened transparent soda drink. Mitsuya Saidaa to the 20th-century Japanese is like Coke or Root Beer to the 20th-century Americans. You grew up with it.
Late February to early March is the season of ume (plum) blossoming. The flowers aren't edible, but during this season citizens prefer to enjoy anything related to ume, such as preserves made from the fruit that cannot be harvested until summer.
What you drank was something that is supposed to evoke the image of ume juice and the sparkling feeling of blossom celebration. The brownish color probably comes from the fact that a lot of locals have the impression that "ume-shu" (old-fashioned homemade plum wine) is brown.
Written vertically on the right-hand side of the label is the name of the drink 三ツ矢梅咲
三ツ矢 means Mitsuya, which loosely translates to "three arrows", hence its famous logo in the center of the label.
梅 means ume.
咲 (-saku) means the verb "blossom", and "ume-saku" is their made-up name that implies the meaning "Plums BLOSSOM". The fact that the letter 咲 is written larger is probably meant to put the emphasis on the "blossoming" of ume which is one of the first trees to blossom after winter. So it's kind of like evoking the "Spring has come!" feeling.
Then under the flowers it says 梅咲き、香りひらく。(ume saki, kaori hiraku.)
which means "plums blossom, fragrance expand." Unlike cherry blossoms, plum blossoms have a subtle fragrance.
On the very left it says 紀州産南高梅 (Kishuu-san Nankou-bai)
which is known to be the most popular type of edible ume in Japan from the region of Kishuu which is now generally Wakayama Prefecture.
By the way, Asahi Soft Drinks Co. (which is different from the beer company) indeed owns the Mitsuya brand now, but I doubt that most consumers pay attention to that fact. When we see the name and logo of Mitsuya, we think "soda".
It would've been ideal if you drank it seeing scenery like these;
https://www.google.co.jp/search?q=%E6%A2%85%E6%9E%97&source=lnms&tbm=i...
