Eh?
Vader never took Luke's saber. Luke had it in his hand when he fell from the gantry, he never dropped it.
As for the lightsabers, in ANH they used special reflective stuff that was originally going to glow on set. It didn't turn out too great, so they replaced it with after touch effects. Thats why some of them appear to be white in certain scenes. On the DVDs, this has been fixed (kinda).
When it came to ESB, they just gave the actors sticks that they'd later paint (and is how theyve been doing it ever since), thats why they looks slightly different.