[Dan_San_Abbott]

Lloyd:
Let's use a little logic. You have just finish streaking the top surface and you want to clean up thje leading edge, what would you use? very simple, the same paint you just painted the top surface with. The would dip it in the paint and turn the brush parallel to the leading edge and the paint forms a line about 20mm wide, and a professional would draw a straight line down the leading edge at full strength. The color is olive brown and is as dark as the cowlings and axle wing.
I think you are making a big deal out of this. Think about it, for what purpose would the black line serve? You would have to wait until the olive paint is dry paint, before you could paint on the black. More time, more money and no purpose.
Lloyd, the finished dark edge is the olive paint applied in concentration and the wing is finished ready to varnish when the olive paint is dry. Look at photo #6 enlargement, the color of the dark line on the leading in some places along the leading edge exactly matches the dark streak on the leading edge and matches the leading edge stacking pad.
Blue skies,
Dan-San