I did it in Painter. I used the oil pastel to lay down a generic pale blue for a base and locked transparency so I could color and blend with abandon without going outside the lines, then put in some streaks of white and a dark grey, then used the Just Add Water blender in Painter to blend them, keeping my strokes going in that same diagonal direction. Then I used the eyedropper to pick up colors from nearby areas, since metal reflects things like that, and put them on, then blended the same way. The blender started picking up black from the outlines, even though I was working on a separate layer, which fortuitously looked good (they're the dark streaks on the upper half of the blade). To finish it, I put a veeery thin white line that you can't see in the jpg along the upper edge of the blade and along the top edge of the line down the middle of the blade.