Bạn dùng toolkit của WPF nhé

http://www.c-sharpcorner.com/UploadFile/mahesh/wpf-colorpicker/

Cách add toolkit

Các dùng ColorPicker

<wpfx:ColorPicker Name="ColorPicker1" Height="30" DisplayColorAndName="True" 
                   Margin="26,20,369,261" />

Code C# lấy color 

ColorPicker1.IsOpen = true;
ColorPicker1.SelectedColor = Colors.Red;
ColorPicker1.ShowAdvancedButton = true;
 
colorPicker2.IsOpen = false;