Salmon are softer and I believe aimed at wet weather/winter riding, black are a bit harder compound, less wear. but not quite as powerful.
I swapped the standard Ultegra pads for Salmon on the front and black at the rear and the combo is far better than the Ultegra.
Wear on the Salmons not bad at all, the stopping power is phenomenal though with good feel. I'd have never done the Ventoux at the speed I did if I had to rely on the Ultegra pads.
I didn't put Salmon on the back as I dont really wish to lock-up the rear-wheel