http://divinetastes.blogspot.com/2007/04/is-color-measure-of-quality-in-olive.html
Perhaps the most important factor determining the color of the olive
oil is the time of harvesting of the olives. This is due to the fact
that green olives have higher chlorophyll content (the green color we
seem to love so much). As the olive matures depending on it variety it
turns something like this:
Mission Variety: Pale Green to Straw (early season), light pink to
dark red, dark purplish, black, light pink to dark red, dark
purplish(late season).
Sevillano Variety: Slight blush stem end, half or more dark red,
straw, slight blush stem end.
As you can see (I wish I had pictures) there are no black olives and
green olives, just olives that turn black after green and sometimes
greenish again –depending on the variety. Yet another misconception
some people have about “black olives” and “green olives” .
Chlorophyll (derived from ancient Greek: chloros = green and phyllon =
leaf) is of course only one of the main pigments in olive oil. Another
main pigment is Carotenoids which provides it with the yellow red
color.
As the olive ripens, the combination of the different pigments such as
Chlorophyll and Carotenoids as well as other pigments changes, from
higher proportion of chlorophyll to a lesser one. As a result so does
that olive oil’s color change. If pressed early in the season, the
olive will be dark green. As the olive matures the color of its juice
changes to a paler one.
There are many more reasons that affects the colors of the olive oil
such as the weather, cultivar and pressing method etc. However, I
believe the season (early/late) to be the most dominant factor in the
color of the olive oil.