Color picker or Color Chooser is an application or component, to pick colors from photos and other images. It can output colors in either RGB mode or hex, which is useful for web designer, UI designer and also for artwork designer.
Here's a list of open source color pickers. In most of the Linux distribution it is pre-installed.
KColorChooser is a color palette tool, used to mix colors and create custom color palettes. Using the dropper, it can obtain the color of any pixel on the screen.
A number of common color palettes are included, such as the standard Web colors and the Oxygen color scheme.
KColorEdit (KDE Color Palette Editor) is a palette files editor. It can be used for editing color palettes and for color choosing and naming. It supports KDE and GIMP palette files.
Gcolor2 is a simple GTK2 color selector to provide a quick and easy way to find colors for whatever task is at hand. Colors can be saved and deleted as well.
4) GIMP Color Selector Tool:
GIMP is an impressively powerful and freely-distributable Adobe Photoshop alternative open-source application for digital image composition, authoring, retouching and processing.
Color Selector Tool is a component of GIMP, it can also be used as normal color selector or picker. But to use this tool GIMP must be started, which is time consuming as compared to other color pickers listed above.