The color of the sort arrow in the header row is actually not black.

It's color #23 from the style guide: http://wiki.magnolia-cms.com/display/UX/Style+guides+for+visual+design#Styleguidesforvisualdesign-Messageslistandgenerallistofitems

Or more blatantly put: it's #3E5900.

Please also make sure that the arrow shows up vertically centered with the text in the header.