The hands are built into the model and since grievous is white they always are white. You have no clue how long it took me to remove, ajust, and resize those hands from the minifig model... Then merge all 4 lightsabers into it then fix all the joints, textures, LODs, ect. UGH! I'm tired just thinking about it...
EDIT: OH! then write out the script with hide nodes for the hands then package it all...