The moment Reniel hugged Giselle, his power was back along with his hair color 
And that's the reason
And that's the reason
Spoiler!
Giselle was able to mend his mana vessel when she poured all her magic in him trying to save him from the stab wound she inflicted on him and then lost her memories.
I like to post spoilers for the stories I find interesting because I don't have much time to read novel due to busy schedule you will find me in most romantic historical manhwa comment sections
