I don't see any fields available for that. I tried all this:
ColorDialog cd = new ColorDialog();
cd.AnyColor = true;
cd.AllowFullOpen = true;
cd.SolidColorOnly = false;
but it probably does something else.
If it impossible to do so with win forms control, maybe there is WPF color picker that I could use in WPFHost element?
The AlphaColorDialog
project can be downloaded from sourceforge at: https://sourceforge.net/projects/alpha-color-dialog/
Source C#
files are in the src
folder in the zip file.