As far as the theory of a lightsaber goes, it would, in fact have to be a blade generated from some sort of plasma. The color of the plasma may even be dependent on the gases used when the blade "ignites", which is also related to the idea of the use of crystals. A widely used term to describe the sound of a lightsaber's blade igniting is "snap-hiss", which returns to the idea of a gas being the source of the blade. The crystals may theoretically be compressed compounds in a solid form, which through a simple electricity-based reaction can release a very small amount of the gas component of the compounds that they are comprised of. Barring the obvious issue of the eventual degradation of the crystals, this would provide a relatively easy to access source of the color-defining gas.
Returning to the issue of the blade being comprised of plasma, it's quite obvious that the integrity of the shape of a "blade" can only theoretically be maintained through extremely powerful and precisely calibrated magnets. The may in fact be included in the fictional component referred to as an "emitter", which is considered an essential component of the lightsaber. The best example to refer to concerning the idea of calibrated magnets and how the emitter is in fact the source of whatever energy maintains the shape of the blade would be the emitter on Obi-wan's lightsaber, which shows a distinct shape more or less akin to a satellite dish, providing the necessary points of magnetic pull against the plasma.
If one were to look at the magnetic field and how it reacts to, say, a shot from the blasters that are also rather common in the Star Wars universe, it would become apparent that both opposing forces are heated and sustained by electricity.This further reinforces the idea that blasters are also capable of firing plasma, as opposed to pure energy.
In the case of both tools, the additional problem arises with the need for a power source. A single lightning bolt, which is known to be a form of natural plasma on Earth, generates over 100 million volts of electricity from its discharge. A lightning bolt, however, lasts less than half second, longer if conditions are right. In order to sustain a constantly present blade, the energy demands would be bordering on astronomical in comparison to our current power generation capabilities. It is shown, both in movies and in the expanded universe, that lightsabers are usually ignited for several minutes to hours at a time. If not that, then they are constantly being turned on and off. This, in addition to the already existing need for a very powerful magnetic field, places a demand on the power source that would suggest that it would be necessary to have a portable-sized battery with an output equivalent to a sun.
With the composition of the blade, it's source of gas, and potential power source already covered, two problems remain which can be linked to one another.
The first being the insane amount of heat pouring off of a blade made of actual plasma. If one were to eventually overcome the need for a battery with the output of a star, a sustainable source of gas to be ionized, and a field to contain the plasma and maintain its shape, they would still have to deal with the heat of a star less than a few inches away from their hand. If another field were to be generated that could contain the inevitable radiation, this problem would be easily solved, more than likely alongside maintaining the shape of the blade. In addition to this arises the need for a metal that could survive extended exposure to the internal heat of the circuitry as well as the inevitable hole in the magnetic field that would need to be present for the blade to be ignited. This may be why it seems that the Star Wars universe also has its own series of industrial grade metals that are capable of repeatedly surviving meteor strikes, fire from weapons, and reentry into planetary atmospheres.
With these issues addressed, I would say that it is sufficient to say that we are still far, far, away from having functional lightsabers. The physics involved are known, but the technology to produce the results on a handheld scale are beyond the reach of modern technology.