As brown as a berry
Entirely or very brown; often referring to a suntanned skin.
From Chaucer's Canterbury Tales, (the Monk's Tale), 1386-1400:
He was a lord full fat and in good point;
His eyen stepe and rolling in his head
That stemed as a fornice of a led;
His botes souple, his hors in gret estat,
Now certainly he was a sayre prelat.
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.
[Note: a palfrey is a horse]
See other 'as x as y similes'.