I selected a color picker for a style editor in Tabs Studio and here is a list of most interesting open source WPF dialog based color pickers that I found (not commercial and not drop-down color picker controls).
Windows common color dialog
First of all there is the common Windows color dialog. Providing consistent user experience since 1990:
Simple WPF Color Picker by Sacha Barber
Sacha Barber wrote the CodeProject article about his WPF color picker and later Mark Treadwell did various enhancements on this code. Still color position is not preserved when switching swatches, colors range on swatches is limited and there are no additional controls to tune color other than transparency.
Color Picker Custom Control from Windows SDK
You can find it in the “c:\Program Files\Microsoft SDKs\Windows\v7.0\Samples\WPFSamples.zip” archive on your machine and read about it in the Uncommon Dialogs: Font Chooser & Color Picker Dialogs blog post. Functional, but requires more polishing for practical use.
Color Picker from SharpDevelop
Open source SharpDevelop IDE contains a color picker with a nice conventional design.
ColorComb by Shawn A. Van Ness
Shawn A. Van Ness developed the ColorComb color-picker dialog with the ability to adjust stylus stroke properties. It is very easy to select a color you have in mind looking at the comb and use the brightness slider for fine tuning. Cells in the comb provide palette like qualities allowing to choose a distinctive collection of colors. Lack of HSV and RGB controls ensures less distraction when selecting a color.
ColoRotate is an online service allowing you to select colors in 3D. It positions itself as the most intuitive way to work with colors “in a way that matches how our minds process color.” The site has an interactive demonstration showing how “our visual nerves register color in terms of the attributes of color: the amount of green-or-red; the amount of blue-or-yellow; and the brightness.”
Colors dialog in Microsoft Office 2007
Colors dialog in Office 2007 has a design very similar to ColorComb plus it has comparison between a new and the current colors.
To be a part of a XAML editor in Tabs Studio, I liked ColorComb most. I removed ink-specific settings from the dialog and added the current color for comparison.
If you want to use this ColorComb modification in your application, following is the download link: ColorPickerSampleSV.zip.