Origin of phrase "Brown as berries"

Where does this phrase come from? It has been used by Dickens (http://www.online-literature.com/dickens/curiosity/15/), Robert Service (http://www.explorenorth.com/library/service/bl-rollstone28.htm) and contemporarily (http://www.rosers.co.uk/newsletter.htm), but how does it make any sense?

Berries are usually red (which may possibly be a clue, given that pale Anglo-Saxons will go more red than brown in the early stages of tanning), but I can’t even name any brown berries!

Here’s a line describing the ‘Monk’ from General Prologue of the “Canterbury Tales”.

207 His palfrey was as broun as is a berye.
His palfrey (saddle horse) was as brown as is a berry.

Then this line from the “Cook’s Prologue”…

4367 Gaillard he was as goldfynch in the shawe,
Gaily dressed he was as is a goldfinch in the woods,
4368 Broun as a berye, a propre short felawe,
Brown as a berry, a good-looking short fellow,
4369 With lokkes blake, ykembd ful fetisly.
With locks black, combed full elegantly.
4370 Dauncen he koude so wel and jolily
He could dance so well and jollily
A lot of Welsh people are relatively dark aren’t they? Maybe some English people were too.

Anyway, this is one of his stock phrases, maybe it was already around before him?

So the phrase was in Chaucer? Good news/Bad news … somebody’s probably done a Master’s on the question … but the answer is probably 'No-one knows".