If Dorian actually thought things through, he'd decline the offer. After all, the portrait's already been messed with via magic before both in this world and his own and things went absolutely horrible then. He's got a part of his soul steeped in such old magic and filled to the brim with so many regrets, it's entirely possible that something weird would happen if Constantine added a little to the canvas.

Of course, Dorian doesn't actually think things through, so when Constantine offers to add a little something to the portrait, he acquiesces.

"Go ahead," he responds, with a little nod.