T HERE WERE NUANCES to an angel's Grace that were rarely spoken of or explained to humans, and went largely unremarked upon by angels because they already knew and what did it matter? It wasn't as if angels were going around leaving huge chunks of their Grace in random humans, after all. Sure, there were the occasional Vessels who retained a little more than the usual fading imprint on their souls from having housed an angel for a brief time, but it had been a very long time since the angels had walked freely among humans and many had dismissed it as a quirk of stronger bloodlines before putting it out of mind.
And then Sam Winchester said Yes to Lucifer, wrestled control away from the Archangel inhabiting his body, and flung himself down into the Cage with Lucifer and Michael along for the ride to be trapped there until a recently promoted to seraph Castiel retrieved his body and Dean Winchester bargained with Death for the return of Sam's soul. That soul, battered and shredded and patched over as it was, retained more than a fleeting imprint of Grace, not just from Lucifer but from Michael as well. When the Wall inside his head broke, the Grace was no longer held back and fused more fully with the soul it was attached to before Castiel came and took upon himself the pain inherent to Sam's flayed soul, allowing the human to rest, to sleep, and to finally heal... and to fully absorb the Grace.
The Trials to close Hell took their own toll on Sam, ripping new furrows into his soul, burning away the toxins of lingering demon blood from months of addiction and use that had been heaped upon the deeply embedded traces introduced in infancy. With the Trials left incomplete, those furrows were left open and raw, and into that ragged and tattered internal structure came Gadreel, calling himself Ezekiel and pouring much of his own essence into Sam in effort to heal him from within even as he bolstered his own ragged and tortured Grace with the light of Sam's soul. When Sam was finally made aware of his body-hijacking passenger and forced Gadreel out, the separation was less neat than might have been the case had Gadreel left more willingly, and so a greater portion of Grace was left behind than usual once more. Castiel assisted Sam in the removal of as much of the Grace as he dared, but in the end he could not remove the parts already fused into Sam without risking Sam's death-- something Castiel would not even consider to be an option, not even when the meager syringe of Grace they had extracted was not enough to track the disgraced former Sentry of Eden.
There were problems. There were challenges. The angels warred and made peace, rose and fell. The Darkness was released, Lucifer escaped the Cage, God showed himself for the first time in the form of the prophet Chuck, Amara and Chuck made peace and left, Lucifer of all angels sired a Nephilim, Castiel was killed... and through it all the Grace in Sam's soul became so deeply fused to his being that it was nearly impossible to tell that there was any Grace at all within the soul's light. When Jack was born, he Imprinted on Castiel as his father, but the first bond he formed was with Sam, soul-Grace to soul-Grace. And then, miracle of miracles, Castiel was resurrected (or perhaps resurrected himself after Jack Reached out to him and woke him up), and in their own careful, private moment he finally had the courage to seize the moment and ask Sam to bond with him as his mate. And so Sam gained a piece of Castiel's Grace to twine around his soul, and that was the tipping point for all the rest.
Grace, by and large, appeared on the visual spectrum to be a bluish white. On the greater sensory spectrum, Grace carried the imprint of the angel from which it came, which included the resonant frequency of the angel's class, their hierarchy, and their Aspect. Lucifer's Aspect had been Water, which had turned to Ice as the Darkness corrupted him from within, and his wings reflected that in their appearance, turning wings that were once a match to Michael's scarlet into the pink and white seen in a flamingo. Michael's wings, too, changed, going from scarlet to copper and rose as the two were pulled further and further apart by a destiny neither of them wanted but could not find the will to fight. Jack's own wings were pale yellow, reflecting his sire's Aspect in the paleness of their color, but most clearly showing his Imprint on Castiel, whose wings before his harrowing trips through Hell had stained them black once shone like burnished gold, nearly as bright and beautiful as the Archangel under whom he had first served before Gabriel had fled from Heaven.
With so much Grace fused to his soul, it only stood to reason that the morning after bonding with Castiel (and consummating that bond for several hours after) when Sam woke up to feel his back aching, it wasn't just from the physical aspect of their bonding. Nor were the wings he could see stretched out over his and Castiel's naked bodies just the single, somewhat damaged pair of blackened wings he had become used to seeing just over Castiel's shoulders. He could even be forgiven the initial alarm upon seeing that second pair of wings that caused him to fall out of the bed with a yelp as arms and wings both flailed about in an awkward and mostly futile attempt to catch himself. Even if the sudden absence of his mate against his side had not awakened Castiel, the noise Sam made would have done it, and a tousled head of dark hair appeared over the side of the bed with wide blue eyes staring down at his sprawled out mate, hard muscle and scarred skin a direct and entirely too enticing contrast to the pristine, newly sprouted sunset orange wings half curled around Sam's prone form.
"Are you okay?" Castiel asked, although he knew the answer. Years of living among humans, and the Winchesters in particular, had given Castiel some insight into why humans would ask these seemingly foolish questions with obvious answers. Lacking the natural telepathy common among angels, the verbal inquiry was as much to alert the person being asked that the one asking cared enough to express concern. As bonded mates, he and Sam shared a mental bond as well as a spiritual bond between Sam's soul and Castiel's Grace, but Castiel was familiar with the contentious history surrounding Sam's psychic powers - contention that Castiel was ashamed to admit he had contributed to under Heaven's orders - and so had decided the night before to utilize the more human manner of communication while Sam became accustomed to their connection.
Now, however, it seemed that he and Sam both would need to become accustomed to more than either of them had previously supposed or even expected, and Castiel was more than a little concerned for how Sam would react. The startlement was probably to be expected, and the lack of screaming or profanity was cautiously reassuring. Then again, considering the way Sam was blinking up at him, it was entirely possible that his beloved was in shock.
"I have wings," Sam said after a long moment, his words slow and precise as if he were testing the fit of them on his tongue. The corners of his mouth twitched down and his lips thinned as his eyebrows drew together in confusion, which seemed to indicate that the words were not making sense to Sam's mind regardless of their accuracy. He tilted his head back a little bit further to meet Castiel's eyes. "I have wings?"
"It would appear so, yes," Castiel confirmed, giving a cautious nod. Sam mimicked him, nodding slowly as he stared up at the sweeping curtains of orange feathers curling around him.
"Well, fuck me," he said at length.
It was a statement of resignation and dismay. Castiel knew that, knew perfectly well that it was not actually an invitation. However, staring down at the glorious expanse of bare skin of the body he had spent several hours worshipping as only one can when worshipping a mate, framed and unintentionally displayed by the curves of healthy, full-plumed wings as glorious as could have been found on any angel before the Apocalypse and all the troubles thereafter, Castiel found himself responding with a raised eyebrow and a deliberately even-toned, "Again? Well, if you insist...."
Sam's resulting blush was just as attractive as the rest of him.