When making biodiesel from different oils from different restaurants the colour of the resulting fuel is either dark or light in colour. I assume this would be the result of the different foods cooked in the oil, and/or the amount the oil has been heated and cooled.
When left for awhile in a container this "colouring" seems separate. When left in an open jar it fades. When in your fuel tank there seems to be a film around the top.

Can this be eliminated by using a neutralising additive?