Wikia

Red Sparks (incantation unknown) is a spell used to shoot a jet of red sparks from the tip of the wand.^[1] It is covered in the book The Dark Forces: A Guide to Self-Protection^[1], and may be related to Green Sparks.

Known uses

In 1992, while searching for an injured unicorn, Rubeus Hagrid instructed Harry Potter, Hermione Granger, Neville Longbottom, and Draco Malfoy to send red sparks in the air if they were trouble, and Neville Longbottom did use it later that night.^[2]
In 1995, champions who were lost in the Third Task Maze would send up red sparks, and when Viktor Krum got knocked out, Harry used the Red Sparks to signal that he was down.^[3]
It's possible that the Order of the Phoenix might have used the spell in 1995 as the signal for the Advance Guard escorting Harry Potter to 12 Grimmauld Place.^[4]