All the colors in the rainbow...

Voilà, a couple of plasma pictures, as promised. All of them are in Argon at 1 mBar of pressure, except the closeup of the stainless steel electrode.

Meanwhile I can even tell you where the colours come from:

The bright blue ones have a brass electrode, and the blue light consists of copper and zinc lines. Pink means the power is not enough to excite metal, so all you can see is the Argon 2p - 3s lines. The bright green pictures are steel electrodes, in the one on the left you can also see the steel glowing white-hot. And finally, the yellow plasma is said to be (according to Fraga et al. 2003) the excitation of a Rydberg state of CF₄ which dissociates into an emitting CF₃* fragment. The CF₄ comes from the Teflon used in this particular electrode. Teflon is (CF₂)_n, and seems to outgas CF₄, leaving behind a Carbon deposition on the electrode.

There you go, all mysteries revealed!