New answers tagged scanning
There's also a number of apps available. I've never tried any to know how well or not they work. Swatchmate Cube was funded from Kickstarter in 2013. I've seen one that's a very high end looking sphere but can't relocate it right now. Pantone puts out their own color capture as well, it's quite pricey though. Pantone CAPSURE. Adobe came out with an app ...
Get your (or borrow a) Pantone swatch book and hold it up next to the wallpaper until you find a close match.
This is very easy in Gimp. Just use the bucket tool to fill the background white, and export. Or if you want to go one stage further, then after that, use the select with the same colour tool, press delete (Making sure that your image has an alpha channel) add a new white layer, merge down and export as a jpg, png or whatever.
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