Source; TvTropes
Basically, this is when the main outfit of a redhead or auburn-haired character is green. This trope can sometimes be used to hint that somebody is of Irish descent, but sometimes, illustrators use it as a traditional pair of complementary colors or because they just feel like it. Bonus points if the character in question has nature-themed abilities.
The trope comes from a long-standing tradition among fashionistas that red hair contrasts best against green or blue attire, which is one reason redheads are encouraged to wear green to make a striking visual appearance. The color they encourage redheads never to wear is pink because the colors clash too much.