I can actually tell you from personal experience that the bodies do NOT take rit dye. The joints do, but the hard plastic bodies just mostly stay the same color. I tried dying a Draculaura purple over Christmas? Her joints were clearly purple, except where the hard plastic had covered them and she ended up looking streaky until I washed her, when she was the same pink as any other one.
So, I wouldn't bother with the dying at all- it just doesn't take for them.