How can I call the Control color, I mean the default forms color?

this.BackColor = default(Color);

Seems to be the color that the form designer uses...

I haven't tested this, but I believe it to be:

this.BackColor = Control.DefaultBackColor;

The System.Drawing.SystemColors class has properties exposing the various system colours, so you can do

this.BackColor = SystemColors.Control;

The full range of properties to access other colours is listed on MSDN.