My broken headphones have different colored wires. Are the red, green and blue wires all copper? The colored wire is wrapped around a white thread.
Yes, all of the conductive stuff inside the cable is copper. However, the copper wires are coated with a very thin insulating layer called enamel.
The enamel can sometimes be removed by rubbing molten solder on it.
If this doesn't work, then you might have some luck rubbing some fine emery paper on it.
As you can see, you don't need to clean your fingernails before you do this.
But one of the problems with headphone cable specifically is that it often contains some thread to reinforce it.
The thread makes soldering the cable pretty difficult. Usually, if I find threads in my cable, I just throw it away.
The inner conductor is usually copper. The outside, the colored stuff (red, green, blue) is usually some kind of plastic. You can carefully remove the insulation from the conductor to reach the copper. Some headphones have very thin wires which easily break when you try to pull off the insulation. Don't tell the other electronics engineers here that I said this, but if you are careful you can remove the insulation by carefully holding it in a flame of a match or cigarette lighter.
If you are planning to try and connect the wires together to make it working again, be careful to not make shorts between different colors as that may kill your amplifier.
I broke the connector off a bluetooth microphone for a car stereo. The wires inside the sheath were red and blue, but happily nestled together...I soaked them in nail polish remover for a few seconds, agitating slightly...then burned the enamel off - carefully.
To my surprise...I was left with slightly carbon-covered copper wire...definitely something I could work with. I soldered the newly "cleaned" ends after tinning carefully at 700 degrees. The result is a serviceable salvage job.