Electrons inside atoms can be excited not only by striking photons or electrons, but also by electric fields. The electric fields can be strong enough to ioize the atom, the freed electrons then become a cause for further ionization processes. As the excited electrons fall down the ground state or as collisions between atoms and electrons occur, energy is lost. This lowering of energy is accompanied by the emission of photons of different wavelengths, combining to give an important color. It is the same electric-field assisted excitation that can lead to the glowing of a fluorescent tube light in the presence of strong electric fields. Here we use a home made power supply (up to 20 KV) to demonstrate this effect.
Note of caution: DO NOT ATTEMPT THIS DEMONSTRATION ON YOUR OWN. Only let your qualified teacher perform it for you.