Not to confuse an already confusing issue, but what do people that are red/green color-blind to do? I don't expect us to design around the minority that are color blind, just wanted to throw that into the mix.
As everyone has previously stated aluminum tends to be lighter and slightly less expensive than copper. It really depends on the ultimate design characteristics that are required. There are many custom design transformer houses that may be able to help you decide. You can try Norlake...