“Brown as a berry” means being very brown and often refers to a good suntan.
Nobody knows the origin of the phrase, but it’s old. Chaucer used it twice in his Canterbury Tales and it was probably common in speech years before it was used in writing.
Here is the relevant verse from the Monk’s Tale:
“He was not pale as a forpined gost;
A fat swan loved he best of any rost;
His palfrey was as broune as is a bery.”
One theory suggests it refers to coffee beans, but that can’t be true because coffee wasn’t brought to England until long after Chaucer was writing. A second theory is that “berry” might refer to a nut, but there’s no proof for that to be found in any of the language authorities.
The Oxford English Dictionary does say that, in the 900s, “berry” was most commonly used of grapes. At the time, the climate in southern Britain was very warm and grapes were commonly grown. But I have yet to meet a grape that could be called brown.
However, ancient writers seemed to emphasize light and dark rather than refer to specific colors. The OED gives the earliest sense of “brown” as dusky or dark. Other languages appear to have a similar approach. In Swedish, brun could mean dark-colored or black, dark red, or reddish-brown. In Old French brun meant a dark shade between red and black. Old High German brûn meant dark-colored. So the horse in the Monk’s Tale may have been brown or black.
It’s still hard to apply “brown” to a berry. Raspberries and strawberries are pink to red, blueberries are blue, blackberries are red when green and black when ripe. Gooseberries are pale green.
So “brown as a berry” still stands today, used for a suntan. No wonder English is such a difficult language to learn!