“…Manwë is dearest to Illuvatar and understands most clearly his purposes. He was appointed to be … the first of all Kings: lord of the realm of Arda and ruler or all that dwell therein. In Arda his delight is in the winds and the clouds … Sulimo he is surnamed, Lord of the Breath of Arda.” — Silmarillion, Valaquenta
Not mentioned in this particular passage, but Manwë has a special affinity for birds, especially eagles, presumably due to his delight in the winds. Thus the winged circlet (I also like the hearkening to the winged helmets of Gondor). I maintain that this affinity for birds is the reason Gandalf has such a good relationship with Gwaihir the Eagle — before leaving Valinor to become Istari in Middle Earth, Gandalf was Olorin, one of the Maiar of Manwë.
All of the Valar took the guise of what they thought the Children of Illuvatar would look like when they finally came. Manwë has the overall look of a Noldorin Elf with dark hair and light eyes, which are both also reminiscent of the sky (this becomes more important when we later talk about Varda, his Queen).
I chose a dark stone in his circlet for a similar reason as the dark hair. Also because there seems to be an ongoing elemental theme with the other Valar, and with Ulmo being water I thought black was a better air color for Manwë than blue.
“When therefore Earth was yet young and full of flame Melkor coveted it, and he said to the other Valar: ‘This shall be my own kingdom; and I name it unto myself!’ … And he descended upon Arda in power and majesty greater than any other of the Valar, as a mountain that wades in the sea and has its head above the clouds and is clad in ice and crowned with smoke and fire; and the light of the eyes of Melkor was like a flame that withers with head and pierces with a deadly cold.” — Silmarillion, Ainulindalië
This particular image isn’t entirely aligned with the vision of Melkor’s first incarnation within Arda, he comes amongst the other Valar in a form “dark and terrible”, but is hearkening back to Varda’s knowing of Melkor before the music and having rejected him. There is some ambiguity as to what sort of rejection, romantic or otherwise. I think the idea of Manwë and Melkor being “as brothers” in the mind of Illuvatar and Melkor’s spurned love ending up with his “brother” is an interesting one. Even though Melkor is the more powerful of the two, it cannot grant him fulfillment of all of his desires. It’s a little melodramatic, and I’m sure has spawned a thousand fanfics, but is not without merit.
Melkor, later Morgoth, is usually associated with the darkness. There is, however, reason to read the 3 “great” Lords of the Valar as alchemical elements which would place Melkor as fire. One big hint for this is the type of Maiar each Valar is associated with. Ulmo, who is unarguably associated with water has Ossë and Uinen, Maiar in charge of the tides and storms upon the seas. Melkor’s Maiar are the Balrogs and Sauron. The Balrogs are all fire spirits, although tainted with darkness — Gandalf describes Durin’s Bane as being “shadow and flame” — however not all of the fire spirits are tainted with that darkness, the sun is held by a Maiar of Melkor who defied him. It’s a little glimpse into what could have been had Melkor not fallen.