Do it yourself. It will be easy
In XWidget it will not be easy, because there is no ScaleImage() or GetPixel() api. Wait, do you want me to do this manually? But what if I change wallpaper every hour? Now I also request OnWallpaperChange event.
I suggest "colorpic", the average user won't pick colors from screen
I also suggest you "gpick" (it's more advanced). Anyway that's another piece of software and many extra steps.
the average user won't pick colors from screen
How do you know? You did some ux research?
Isn't RGB doing the same thing? Adding another fancy name is useless
Where is RGB for image controls? There's a bunch of controls with images.
There is an image mask in some control! (I never use it)
WOW! Try to clip fill & gradient with that mask.