Einar,

You need to provide a string as the second parameter for the DefaultValue attribute.

[Category("Appearance")]
[Description("Specifies the colour of the controls border.")]
[DefaultValue(typeof(System.Drawing.Color), "255, 0, 0")]
public Color BorderColor
{
   get { return _borderColor; }
   set
   {
      if (value != _borderColor)
      {
         _borderColor = value;
         base.Invalidate();
      }
   }
}

The example above specifies a default colour of Red.

Regards
Neil