In Revelation 12, Satan is depicted as a red dragon with seven heads and ten horns. But why is Satan represented as a red dragon? As we've already explained, the seven heads represent the evil angels who ruled over Babylon, Persia, 4 Greek countries, Rome, and the ten horns represent the angels who ruled over the ten nations that separated from Rome. You can read more about this in the links below.

http://godnara.egreef.kr/bbs/board.php?bo_table=07_01_eng&wr_id=309 

When Satan is depicted as a red dragon, God is showing us that Satan was a high ranking angel. The purple and red clothing in the Bible is only worn by those in authority. In the history of Daniel, there is an incident where Belshazzar, the king of Babylon, poured out wine from gold and silver vessels in God's temple, praised idols, and a human hand appeared and wrote on the wall. Surprised by this, the king told the man who interpreted the meaning of the letters that he would dress him in purple and make him the third ruler of the country, indicating that purple is the color of clothing worn by those with high authority.

[Daniel 5:7] The king cried aloud to bring in the astrologers, the Chaldeans, and the soothsayers. And the king spake, and said to the wise men of Babylon, Whosoever shall read this writing, and shew me the interpretation thereof, shall be clothed with scarlet, and have a chain of gold about his neck, and shall be the third ruler in the kingdom.

Similarly, the harlot represented by Babylon in Revelation 17 is dressed in red, which is also a symbol of high status.

[ Revelation 17:4] And the woman was arrayed in purple and scarlet colour, and decked with gold and precious stones and pearls, having a golden cup in her hand full of abominations and filthiness of her fornication:

So the depiction of Satan as a red dragon with seven heads and ten horns is a representation of the fact that they were angels in a high position in heaven.