The quality is due to the fact they filmed it on 35mm B&W Film, it was colorized in the 90's but I suspect it's been redone since it looks really good.
I've seen the episode before (actually just regular shows stitched together with some new footage) on DVD but it was in B&W.
They colorized the early B&W episodes of Bewitched too, the technology has come a long way since the early attempts!
There is some real color footage of the show here:
https://www.youtube.com/watch?v=OxsiItd1iN8
An audience member snuck in a movie camera and filmed them shooting.