Though their bodies must be contained in vessels when they come to earth, angels can still see echoes of each other's wings. They're as insubstantial as shadows, translucent enough to see through and past, but they're there.
None of the angels ever talk about it, but Castiel has always been convinced that an angel's wings suit his personality. Most of his garrison wear wings of white, gray, and brown. They are good, solid colors for good, solid brothers and sisters.
Uriel's were black. The uncommonness of them made it easy for the others to spot him, made them feel more comfortable following his lead. Until he killed them for refusing his side of the war, that is.
Castiel's are even blacker than Uriel's. Here on earth, he stands before mirrors, staring at them. Uriel used to be a sign that his suspicions were wrong, that the darkness of his wings had nothing to do with the course of his destiny. Then Uriel rebelled and Castiel followed suit. The paths they chose may have been different, but neither followed the orders they were given from on high. As the world moves closer to all-out war, Castiel worries more and more about what his wings mean for him.
When he served under Anna, her wings were a bright, warm red, the color that her hair was when she fell and took a human form. When next he saw her, after she reclaimed her grace, they were pure fire. Heat followed in her wake, made Castiel desperate for her approval.
He's never met Michael-most angels haven't; he seems to be as elusive as their father-but the others tell stories, and he believes them. He believes that Michael's wings are a white so pure that they glow, believes that the white feathers give way to a deep, blood red along the edges. He sometimes looks at Dean and imagines those wings unfolding from his back, knowing that of all the angels he could have been a vessel for, Michael's wings suit him the most. Castiel also knows that he never wants to have to see those wings sprout from between Dean's shoulders.
Raphael's wings are a deep purple, the color kings would take for their robes. Whether the conceit came from the archangel or the mortals first, Castiel didn't know, but it suited them both. His brothers always said that Raphael lacked mercy, and Castiel found that out first hand at Chuck Shurley's home, when he recognized the purple wings and knew his end was coming. Castiel associates that particular color with death, now.
Gabriel has wings of brightest gold. They shine with a light Castiel would recognize anywhere, even though he had never seen his brother before having to save Sam and Dean from his hold. Humans may not know the difference, but Gabriel cannot hide from another angel's eyes.
He's always heard tales that Lucifer's wings turned skeletal during his fall, that Michael's light blasted the feathers right off the bones. Castiel never believes those stories the way he does the other things he hears. He never really knows why until he comes face-to-face with the devil, when the truth that he had known for centuries is confirmed.
Lucifer's wings are as black as Castiel's own.