The Jedi in question is Ki-Adi Mundi, and I noticed this when watching as well. A while later, when he's standing in the Republic Gunship with Mace Windu, his saber is blue again. There's a slight possibility that there was another Jedi very close to him and it was that Jedi's green saber we saw, but I'd have to examine the scene very closely to be sure.
Good catch! The Jedi you are referring to is named Ki-Adi-Mundi. His lightsabre is supposed to actually be a light purple-ish colour, so not only is it a slip-up that the colour changes, but both colours are wrong!
Actually, Hermie Odie, you're wrong about his light saber being purple. The purple saber belongs to Master Windu. Master jedi Ki-Adi-Mundi's saber is blue, but magically turns green in a matter of two seconds? That's not long enough for him to pick up a fallen jedi's saber. Maybe he a magic jedi?