For duron, the color of the die makes no difference b/c all durons are made in Austin and are aluminum...HOWEVER, for t-bird, that's another story...although it's hard to identify copper t-bird and aluminum t-bird just base on the die color...from all the reports...it looks like if your t-bird core is blue...the chance are pretty good that you have a copper t-bird...and looks like copper t-bird has the following three types of code...AEEA, AHEA, or AJEA...so if you see a blue t-bird with one of the three code, i would say the percantage it is a copper t-bird is pretty high...