Yes, it is always negative.
It is used to say that someone pretended to be something they are not. Their true intentions or defects are shown by some act they committed. I think it comes from old falsified currency that were pieces of copper covered in a thin golden layer and they paid with them as if they were gold. A little scratch, and the copper core showed up.
- ¿Así que sólo querías dinero? Qué pronto enseñaste el cobre.
So you only wanted money? You showed soon your true intentions.
- Luego de que la denunciaron, la presidenta municipal mostró el cobre y amenazó a los periodistas.
After having been accused, the mayor showed her true colors and threatened the journalists.
- Yo siempre creí que Pedro era decente, pero anoche en la fiesta sacó el cobre y todos queríamos que se fuera.
I always thought of Pedro as a decent person, but last night he behaved so bad that we all wanted him to leave.