Wand Sparks – Harry Potter Lexicon

"Now, if any of us finds the unicorn, we'll send up green sparks, right? Get yer wands out an' practice now -- that's it -- an' if anyone gets in
trouble, send up red sparks, an' we'll all come an' find yeh..."
-- Hagrid explaining the use of red and green sparks in the Forbidden Forest

Wand sparks are red or green sparks emitted from the wand, used as a signal.

References from the canon

Harry, Draco, Neville fired red sparks into the air to call for help in the Forbidden Forest (PS15).
Harry and Ron shot sparks to hold off an advancing skrewt (GF21).
For the third task, each champion was instructed beforehand to send up red sparks if he or she got into difficulty and wished to be rescued (GF31).
When Harry's temper was getting the better of him, his wand inadvertently gave off a few red and gold sparks (OP2).
The signal that it was clear for the Advance Guard to leave Privet Drive with Harry was a shower of red sparks, then green sparks, far off in the night sky (OP3).
Harry's wand spun around by itself and shot gold flames at Voldemort when the latter pursued him after his departure from Privet Drive. Albus Dumbledore later said that this was because during their duel three years before, Harry's wand had taken on some of the power and qualities of Voldemort's wand, recognized him as both kin and mortal enemy, and regurgitated some of his own power back at him (DH4, DH5, DH35).
Harry's wand emitted a bang and red sparks when he closed the curtains over Mrs. Black's portrait (DH9).
Red Sparks and Green Sparks are spells listed in The Dark Forces: A Guide to Self-Protection by Quentin Trimble (Pm)