The Product Price is set to 0.00 Special is set to 20%

The Product Priced by Attribute is set to YES

The attribute prices are defined without the price prefix of +

The Display Price is made up of the lowest attribute price from each Option Name group.

If there had been a Product Price, this would have been added together to the lowest attributes price from each of the Option Name groups to make up the display price.

The price prefix of the + is not used as we are not "adding" to the display price.

The Colors attributes are, their prices before the Special discount is applied:
White $499.99
Black $519.00
Blue $539.00

The Specials Price is a flat $100 discount. This $100 discount is applied to all attributes marked attributes_discounted Yes.