The electrosaber is essentially a hilt with a collapsible metal baton for the blade. This of course means that the blade is not pure energy; however, it is the closest thing we have to a lightsaber similar to what we all have in our minds.

In theory, when one flips a switch the baton would shoot out (possibly through an array of magnets or electromagnets), and then a high voltage would run through the baton, ionizing the air around it. This would (in theory) create a glow around the baton, producing a beam- like effect, possibly being able to cut through objects.

A prototype has been built (not by myself), and the person who built it reported that he achieved partial success. He used a copper pipe instead of the baton, and it did produce some glow @ 10kv; this could not be seen in broad daylight, but it could be seen in a dark room. He have done other experiments, however, and found that 50kv produces clearly visible plasma. See the link below for his site and some pictures.

As for colors, it would range anywhere from violet to white, depending on the voltage; lower voltage would be violet, while a higher voltage would be white. This is actually similar to lightning -- picture lightning on a stick, and you won't be too far off.