Inkscape and The_Gimp should share a colour picker. The Gtk one is simple but not powerful enough for serious work.
Of course, some of what pickers and features we need will depend on what exactly we mean by [InkscapeColor 'Color' in Inkscape]
Here are some example pickers that are worth copying:
And a few recent additions
There is a Adobe Patent on Tabbed Palettes.
It would be best to avoid copying any of these exactly.
I like the idea of allowing the user to adjust groups of colours in a picture simultaneously.
The colour picker should be implemented using a widget that displays SVG.