Why are my olives discolouring in their brine?
I was going to answer, but while searching for the name of the product used to make the olives black I found this perfect answer from http://www.oliveoilsource.com/asktheexpert/are-olives-dyed-make-them-black

Olives naturally turn black as they ripen. When unripe they are green.
As they ripen they get reddish, then purplish and finally black.
&quot;Ripe Black Olives&quot; in a can are actually olives which are neither
black nor ripe when they are picked.  They are picked very green and
then cured using dilute brine and lye solutions.
Lye treatments cause natural phenolic compounds in the olives to oxidize to a black color.
Calcium chloride salts, iron salts (ferrous gluconate) and compressed
air bubbled through the curing vats help develop the black color. So
there is no black dye used but the olives are treated to make them a
nice uniform dark black.
Home curing of olives without these added salts will result in mottled
and brownish olives which taste just as good but are not as
attractive.
The opposite tact is used to make a nice green California- style
green-ripe olive. Air containing oxygen must be excluded to avoid
natural oxidation and darkening of the skins.