Further to the question of meaning: I see now that
cré is synonymous with
giz 'chalk'. Now, in French the word for this substance is
craie. So maybe the proverb is simply some kind of Franco-Portuguese calque (no pun intended):
craie avec craie, lait avec lait >
cré com cré, lé com lé
'chalk with chalk, milk with milk'.
There's always been plenty of vernacular coming-and-going between Spain, Portugal and France, especially the South of France.
Only a hypothesis, but it has this advantage that the proverb makes perfect sense if I'm right