I was hoping Jack would answer.
Comeon kids, copper? Barium? <pained sigh>
Look at the image and apply a little basic physics. From right to left, it clearly gets hotter from the color of the trail. I know it's not a calibrated color camera but it does give a feel.
What seems to happen when it turns green is the peak output of emitted energy is higher than what is visible so you see not the UV itself but it's flouresence in the surrounding gasses fo whatever reason is green, mainly a specific oxygen green wavelength I'd guess.