Promise of Rainbow/Rulings

Japanese FAQ Rulings (Source)

 * Q2704: My deck only has ten cards remaining, and because of my opponent's card effect, all ten of them will be milled, the fifth card is Promise of Rainbow, can I return it to my hand and cast it so my remaining five cards won't get milled?
 * A: No, it can only be added to your hand after all ten cards are milled by the effect. Until the effect that mills all ten cards of your deck is resolved, the effect remains on standby, and you cannot activate any other effects until the first effect is resolved first.


 * Q2705: When two of my opponent's Light Ray ​Black Death Dragon, Abygale is destroyed during my turn, one of the Abygale's ability activates and mills three of my deck cards, and one of the cards milled is my Promise of Rainbow card, but before the second Abygale activates its ability and mills three more of my deck cards, can I activate Promise of Rainbow's effect when milled and return it to my hand?
 * A: Yes, you can. If multiple abilities are activated at the same time, the turn player takes first priority. In this case, when the first Abygale mills three of your deck cards, and one of them was your Promise of Rainbow card, since it is your turn, you can activate its effect and return it to your hand before the second Abygale's ability activates, and you can also use it to counter the second Abygale's ability, protecting your deck cards from getting milled any further.


 * Q2186: When two of my opponent's Light Ray ​Black Death Dragon, Abygale is destroyed during their turn, one of the Abygale's ability activates and mills three of my deck cards, and one of the cards milled is my Promise of Rainbow card, but before the second Abygale activates its ability and mills three more of my deck cards, can I activate Promise of Rainbow's effect when milled and return it to my hand?
 * A: No, you cannot. If multiple abilities are activated at the same time, the turn player takes first priority. But in this case, since it is not your turn, the timing that activates when your opponent's Abygale is destroyed is resolved first, and only after the abilities of both Abygale's have been resolved, then you can return it to your hand.