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“The face of Elrond was ageless, neither old nor young, though in it was written the memory of many things both glad and sorrowful. His hair was dark as the shadows of twilight, and upon it was set a circlet of silver; his eyes were grey as a clear evening, and in them was a light like the light of stars. Venerable he seemed as a king crowned with many winters, and yet hale as a tried warrior in the fullness of his strength. He was Lord of Rivendell and mighty among both Elves and Men.” - The Fellowship of the Ring.
“He was as noble and as fair in face as an elf-lord, as strong as a warrior, as wise as a wizard, as venerable as a king of dwarves, and as kind as summer.” - The Hobbit.
“So like was she in form of womanhood to Elrond [...] Young she was and yet not so. The braids of her dark hair were touched by no frost and the light of stars was in her bright eyes, grey as a cloudless night; yet queenly she looked, and thought and knowledge were in her glance, as of one who has known many things that the years bring [...] Arwen, daughter of Elrond, in whom it was said that the likeness of Lúthien had come on earth again; and she was called Undómiel, for she was the Evenstar of her people.” - The Fellowship of the Ring
“I believed you to be indeed Lúthien Tinúviel, of whom I was singing. But if you are not she, then you walk in her likeness.” - The Return of the King, Appendix A, ‘The Tale of Aragorn and Arwen’.
“Keen, heart-piercing was her song as the lark that rises from the gates of night and pours its voice among the dying stars, seeing the sun behind the walls of the world; and the song of Lúthien released the bonds of winter, and the frozen waters spoke, and flowers sprang from the cold earth where her feet had passed [...] the woods echoed the name.” - Silmarillion
Lay of Leithian:
“Light of stars was in her hair, and in her raiment glimmering” - note that the physical forms of Valar and Maiar are compared to raiments, so it could be her very form that glimmers in the dappled starlight.
“Her hair like shadow following” as she moves.
When Beren saw her, “enchantment healed his weary feet”
“Feet as light as linden-leaves”
“At her feet was strewn a mist of silver quivering” as she dances under the moon.
“Her song released the sudden spring, like rising lark, and falling rain, and melting water bubbling. He saw the elven-flowers spring about her feet, and healed again.”
“In his arms lay glistening. As Beren looked into her eyes within the shadows of her hair, the trembling starlight of the skies he saw there mirrored shimmering. About him cast her shadowy hair and arms like silver glimmering.”
“grey as evening were her eyes [...] dark as shadow was her hair.”
So, Arwen is apparently both the spitting image of Lúthien and a perfectly genderbent Elrond, so we can assume that most of how they are described also applies to Elrond. Therefore:
Elrond’s hair is dark like shadows and twilight, probably braided, somehow contains starlight, floats behind him when he moves and surrounds him very romantically when he lies down.
His eyes are grey, compared to clear evenings and cloudless nights, and bright and sparkling with starlight.
His face is noble and fair, his skin is flawlessly smooth white or silver; he seems to glisten or glimmer, and moves lightly, and flowers sprout at his feet, and mist swirls around him when under the moon.
He seems both young and old, ageless and filled with memories both happy and sad.
His singing is so heart-breakingly lovely, it draws to mind larks, rainfall and bubbling water, bings hope at the edge of the Void and rises amongst doomed stars; it turns winter to spring, calls bodies of water to respond, and heals all weariness.
He’s a noble lord, venerable as a king, thoughtful, experienced and wise. He’s a mighty warrior in the height of his strength, and most importantly, as kind as summer.
Note that Arwen and Lúthien aren’t described as kind - oh Arwen is queenly and Lúthien turns winter to spring, but Elrond alone is kind as summer.