There are two models of the Double-Auto:
The standard steel receiver known as the Double-Auto, and;
An aluminum receiver known as the Twelvette.
The latter came with anodized receivers in a variety of colours, but most were black.
The standard receives far better prices than the Twelvettes, though some sellers seem to think they are something special.
A standard may be worth between $550-850 depending on barrel and condition. A Twelvette will run $350-650 depending on the condition (anodized receivers are easily damaged), though some of the colours appear to be rare and may be worth more.
My nickel's worth.