User blog comment:ValdemarFIN/READ BEFORE YOU EDIT NEW ORES!!!!/@comment-6339252-20131002121400

I'm not sure, but is the palette swap random?

In my current 1.2 world, I have Copper, Lead, Silver, Gold and Demonite.

I've only come across one of the alternate ores so far.

As such, I think the swap is random, encouraging players to make multiple worlds, in an attempt to get the whole set, as it were. I know I will.