HYJR for hydrangea and HYJK for hijack.
Context: Two of my friends have OCs born from a different franchise. They once paid me to write a fanfic of their characters and after that those OCs more or less became muses in a way. All 3 of us are RWBY fans to some degree and a conversation came around to a RWBY fan fic of the OCs. All this just to say the character names absolutely do not follow RWBY naming conventions lol. Including my own character, the possible initials are (h,g), (y,b), and (j,m), since last name initials can be used as well. After a lot of thought I came to these two names. Well, I came up with a lot more but these two are the final contenders.
HYJR would reference hydrangeas and match the colors of the team leader, but it's a bit of a stretch to get hydrangea from HYJR.
HYJK sounds very much like hijack and so the connection is obvious, plus it sounds cool, but the word doesn't really call to mind any colors. Then again, 3 of 4 team members don't have colors for names, and the fourth one will have whatever name fits the team name.