RealColorPicker

You can choose from over 2000 existing colors, and get in the Clipboard ready piece of code in c #: C# definition, SolidColorBrush, definition in XAML, XAML Brush, definition in HTML; or a small picture.
Also: gradients, and transformations: Complementary, Triadic, Tetradic, Monochrome, Analogous, SplitComplementary and Square