You need a greyscale transparency map, i.e. an image map where the part you want to show as white and the transparent part as black. For your example, the pattern should be black while the solid color should be white. You plug this map in the transparent node, and turn that node to 1. If you want only seethrough effect, not completely transparent, play with this value, or make your black part more greyish.

Hope it helps :)

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