I have the Jumbo Bracelet in PM and it fits perfectly and has never come off. It doesn't sit lose on my wrist though (not too tight either, but just right).
I have the necklace and when I first got it, it did in fact unhook itself once or twice but now it seems to 'know' my neck and doesn't come off either.
My bracelet is in black, there are no problems iwth the colour coming off. The necklace is in natural.
I have the Hapi as well but am thinking about getting rid of it as I am not getting as much enjoyment out of it as I was hoping to.