The bit you see is the heated air. The characterstic color is either white simply because it is hot enough to be the same temperature as the sun (or near enough for our eyes) and so looks white.
Or sometimes green because it has vapourised some of the copper wire and copper emits a green wavelength when it burns
Oops I was wrong.
Air isn't dense enough to give a noticeable black body emission from a plasma.
Most sparks appear white simply because they are bright enough to saturate your eye's color sense.
The blue color is from the emission lines of oxygen/nitrogen molecules in the air, but green is from the copper.