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This is not sarcastic, I'm asking this in all sincerity... why wouldn't it be easier to paint the yellow line in with a brush, spatter with black, then back with the blue? It would still be a lot of work painting all those lines in but it seems to me like it would be easier than trying to cleanly scrape colors away.

Edit: the more I look at the picture, the more I'm convinced on the paint them in idea. It looks to me like a perfect example is being held in the foreground. Some in the background look like they have areas of heavier yellow in the line that could have been a start and stop point with the brush.