The End was wonderful. There were many things that have the fangirl inside of me just beside herself, waiting to see what comes of this. I couldn't wait to figure out what to meta about - because something about this just screamed for meta. I'm going with color and side of a rose.
Current!Dean wears blue. This isn't terribly unusual. We've seen both Sam and Dean in blue before. What makes a difference here is, first, Dean wears blue for the entire drama of the main arc of the episode and, secondly, no one else wears blue. Dean's shirt is a rich blue and considerably more saturated than the colors worn by the other characters.
Blue has got a hefty background, but I'll stick to traditional Western and Christian symbolism. In Catholicism, interestingly enough, blue is Mary's color. It's also the color for heaven and, occasionally, the Holy Spirit (although red is far more traditional for the latter). In art - when it's not depression and the ilk - blue, especially in Christian religious art, stands in for purity, truth, and and clarity of spirit. More generally, blue is considered a "spiritual" color, for whatever weight you give that word. On a "the more you know!" level, I also learned that dark blue is an acceptable color for Advent - or the season wherein Christian await the coming of Christ. Biblicly, blue is also a color for holy places (this should be obvious: Dean ought to be holy, having been raised from the dead as a house for an angel).
Lucifer-Sam, of course, wears white. He not only wears white, but it's a shining white suit that appears to give off an almost immaculate glow against the ruin around him. The pure white of Lucifer's suit is only suitable against the saturated blue of Dean's shirt. Neither Lucifer's suit and Dean's shirt share hue, family, or texture with their surroundings, but they do appear to compliment each other in these areas.
White, in Western symbolism, is purity. It is the color of innocence, light, holiness, and perfection. Along with blue, white is the color of heaven. Red, white, and blue are the traditional colors of the Virgin Mary in art - white for her purity (in Catholicism, she is also free from sin), red for her suffering, and blue for heaven. White is also the color of Easter, the celebration of the rising of Christ (opposite Advent, the coming of Christ) and of Christmas, the birth of Christ (what Advent leads to). Biblicly, white is also associated with righteousness, an interesting choice for Lucifer.
Lucifer toys with a rose as he speaks to Dean. It is a beautiful rose, flowering amid the ruin of the human and demon-wreaked terrain. To quote the University of Michigan website, a red rose "is beauty and perfection, happiness and grace, yet it is also sensuality and seduction," which is an apt description of Supernatural's Lucifer . In very early Christian iconography, the rose symbolises both Christ and Heaven. In Dante's Divine Comedy, the red rose is the symbol of the "army" of the saints. The red rose is the sign of martyrdom - of both saints and Jesus - or the willingness to suffer, even unto death, for God.
Hearkening back to Christian art, the rose is a Marian symbol - or symbol of Mary. To quote a
website on Renaissance art symbolism, "In reference to this legend the Virgin Mary is called a “rose without thorns” because she was free from original sin." If you check out the screenshots in this post, Lucifer's rose appears to be, quite literally, a rose without thorns. (In Paradise Lost, Milton writes of a rose without thorns as well. For Milton, before the fall of man, roses did not have thorns. (There were no troubles.))
So the dominant colors of the episode center on current!Dean and Lucifer!Sam. Quite literally, the other characters blend into the background. (And for good reason - only Lucifer and current!Dean don't have reason to be fighting for survival.) The heart of the actions centers on them as well - and their colors are blue, white, and a red rose. Heaven, purity, and suffering, if you will. Or the Marian colors. I find it somewhat intriguing that Dean, who has suffered in hell, wears heaven while Lucifer, who is also the Rebel and First Sinner, wears purity and, despite the fact that Sam and Dean suffer impossibly in this series, suffering is outside of them - but also, that the suffering - the red rose - is thornless, or sinless and worthy of heaven.
Superficially, neither Dean nor Lucifer should have either of these colors. Dean is actively rejecting heaven and Lucifer is the epitome of the fallen and, beyond that, tempts others into falling. Neither is sinless as the thornless rose indicates, even if everyone in the Supernatural world suffers. Sam and Dean are martyrs for each other, but not for God - lest, as the man outside Dean's motel room suggests, God is love. If God is, literally, love (rather than love is God manifest or symbolically or something), then, indeed, Sam and Dean are martyrs - for love of each other. And Lucifer, too, is a martyr, if he suffers for loving too much and wrongly, not having enough love for humanity.
Still, even if that explains the red thornless rose, Lucifer's immaculate suit and Dean's blue shirt - colors of Mary's clothing in the Catholic Church - remains at a loss. They are unlike each other, yet they are the stand-out colors of the episode. Together they are Marian colors, but Dean is about as unlike the Virgin Mary as one could get and my mind quavers at the thought of equating Lucifer and Mary, though Mary possibly loved God as much as this Lucifer claims to do. Perhaps, though, that's it - both Dean and Lucifer have an intense purity of love. It is what cast them into Hell - Dean's love for Sam and Lucifer's love for God - and perhaps what could also be their redemption?
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If you are curious, Zachariah wore black and (current) Castiel wore brown in this episode. Black, generalising here, can be seen as a negative color - death, Hell, destruction. Brown is earthy, ordinary, and more comforting than black. Everyone in the encampment wore browns and olives reminisce of camouflage and I suspect were supposed to be reminisce of soldiers more than comfort, as Castiel's brown may be.
Because I can't resist here's a huge
page on why the Virgin Mary is a rose and blahblahblah stuff only
chasingtides finds interesting. Also, coming soon: Queering of Lucifer As Christ and A Supernatural Road Trip through American Masculinities.