Cathe's green band would be too light a resistance for the pulldowns, IMO (unless you are used to very light weights, but if you are doing P90X, I assume you don't want to do light weights!).
The B-line bands that Beach Body sells are excellent, but their door attachment bites the big one! It does not seem secure at all.
I bought the heaviest set of three bands from BB, along with a great, secure door attachment from
www.sissel-online.com that secures safely at the top of a closed door, so you can kneel on the floor in front of the door and do pull-downs. (I had gotten the BB door attachment, and after deciding it didn't look secure enough, found the Sissel one).
When I first got P90X, I didn't have the bands or any way to do pull-downs or pull-ups/chin-ups, so I substituted with heavy one-arm rows (with hand and bottom leg on a bench--my step set up at 16": I have lots of risers!) alternating sides with each set, and heavy pullovers.