Contrary to movies like “Giant,” crude oil is not black. Very often, it’s a pale green color. If exposed to air, various components like gasoline evaporate away, leaving a dark-brown sludge that can appear black.
Crude producers like Conoco and Sinclair often painted their tanks, trucks, and other equipment green so that they wouldn’t look so filthy from spilled crude oil. The green was then incorporated into their respective logos. In fact, their is (or was, anyway) a shade of paint called “Conoco Green.”