KColorChooser
KColorChooser is a free and open source small and simple program for mixing colors and creating custom color palettes
ColorMate
ColorMate is a free and open source small app for converting color between three notations #rgb, rgb() and hsl() for Linux
Picturama
Picturama – free and open source, multiplatform software for organizing and viewing images
Photoflare
Photoflare – free and open source, fast and simple, but powerful cross-platform image editor
LazPaint
LazPaint – free and open source, cross-platform light editor of images with multiple tools and filters
LightZone
LightZone – free and open source, multiplatform digital darkroom software and professional-level photo editor
Colorpicker
Colorpicker a free and open source cross platform desktop software for determining the color on the screen
ColorPicker App
ColorPicker App – screen color picker application for Linux Desktop
sK1 Color Picker
sK1 Color Picker – advanced color picker application with magnify glass and palette management functionality
Gpick
Gpick is an free and open source advanced color picker and palette editing tool for Linux