Sauron was a Maia, an Ainu. The Ainur usually took the form that represented their characterization. For example, X character is associated with sky and wind, so his eyes is blue and his hair white. Sauron is most associated with Fire. Therefore, the fanarts picturing him as a redhead.
But Sauron could change his whatever hair color and his appearance: "he could assume many forms, and for long if he willed he could still appear noble and beautiful, so as to deceive all but the most wary." - Of the Rings of Power
According to Nature of Middle-earth, it was Sauron who caused the Fall of Men. In Morgoth's Ring, Andreth describes the guy who caused the Fall as such: "he was clad in raiment that shone like silver and gold, and he had a crown on his head, and gems in his hair."
we see Sauron has terrifying eyes that terrorizes the heart of his enemies. "Now in that hill was the abode of one most evil; and the road that from Beleriand thither came he watched with sleepless eyes of flame". His eyes is like to flames, definitely a result of his spirit being greatly associated with fire. "Then his flaming eyes he on them bent, and darkness black fell round them all. Only they saw as through a pall of eddying smoke those eyes profound in which their senses choked and drowned."
Sauron was a diverse shapeshifter. He could shapeshift into many various things. "From shape to shape, from wolf to worm, from monster to his own demon form, Thu changes," "A vampire shape with pinions vast screeching leaped from the ground, and passed, its dark blood dripping on the trees".
"Sauron should be thought of as very terrible. The form that he took was that of a man of more than human stature, but not gigantic" "a commanding figure of great strength of body and supremely royal demeanour and countenance." L.246
"Upon that ship which was cast the highest and stood dry upon a hill there was a man, or one in man’s shape, but greater than any even of the race of Numenor in stature...And it seemed to men that Sauron was great; though they feared the light of his eyes. To many he appeared fair, to others terrible; but to some evil.”" HoME 5
After the Downfall of Numenor, and Sauron's death in the ocean, he lost his "fair form". "... he wrought himself a new guise, an image of malice and hatred made visible; and the Eye of Sauron the Terrible few could endure." - Downfall of Numenor
According to Fellowship of the Ring, after the Downfall of Numenor, Sauron's skin was black: "the heat of Sauron's hand, which was black and yet burned like fire, and so Gil-galad was destroyed"
In Of the Rings of Power, Sauron abandons his body after Isildur cuts off his ringfinger. "'...he forsook his body, and his spirit fled far away and hid in waste places; and he took no visible shape again for many long years.'"
Tolkien does not mention how long it took for Sauron to recreate another body for himself (he only says it was much much longer than the previous time). But the process began in the year 1000 of the Third Age. Four centuries later he reoccupied Dol Guldur and his shadow became so great that Galadriel was triggered into forming the White Council. In 2850 Gandalf confirms that Sauron has taken shape. So Sauron had taken shape some time before 2851.
In TTT we again see the mention of Sauron with black skin. Gollum says he has seen the Black Hand of Sauron, and that Sauron only has four fingers on his hand. This is definitely because Isildur had cut off Sauron's ringfinger. Sauron was never able to grow back his finger.
So we learned that Sauron had fiery eyes always, he was very tall, he could shapeshift into 'demon'/vampire/werewolf/beast/serpent forms, he could appear to look kingly or beautiful and fancy until he lost his shapeshifting powers, and eventually he became black in skin and hideous and lost one of his fingers.