Gpick is an advanced free color picker and color harmony app.

http://libregraphicsworld.org/blog/entry/gpick-0.2.5-released-with-more-color-tools-and-better-interface

http://libregraphicsworld.org/blog/entry/creating-color-schemes-with-gpick

Features:

    Create palettes from images
    Color scheme creation tools
    Fast color picking
    Magnification
    Colors automatically named
        Closest color name is found using CIE94 color difference function, which is closer to the way humans interpret color differences, than difference of red, green and blue color amounts 
    Export to following formats
        Inkscape/Gimp Palette (*.gpl)
        Adobe Swatch Exchange (*.ase)
        Alias/WaveFront Material (*.mtl) 
    Import from following formats:
        Inkscape/Gimp Palette (*.gpl)
        Adobe Swatch Exchange (*.ase) 
    Picked colors can be copied to clipboard in multiple formats
        Hexadecimal notation - #rrggbb
        Functional RGB notation - rgb(red, green, blue)
        Functional HSL notation - hsl(hue, saturation, lightness)
        Custom conversions can be done, but requires some Lua language knowledge 
    Oversampling with five falloff types
        Multiple pixel values are averaged to help picking colors from sources with noise, patterns and etc, where each pixel is different and overall color is produced by different amounts of particular color 
    Mix two or more colors using variable number of steps and different mixing types
    Create lightness and/or saturation variations
    Create harmonious colors
    Create color schemes 

Reproducible: 

Steps to Reproduce: