“red.” Indicating war, vengeance, and bloodshed.
“myrtle trees.” The myrtle trees stand for Israel. The myrtle “tree” (sometimes called a “shrub”) is an evergreen tree that can grow to 20-25 feet, usually in cooler, wetter areas of Israel. It used to grow on the Mount of Olives, but it no longer grows there, but it is now used as an ornamental shrub in Israel. It has a pleasant odor and delicate white flower, and its branches were used as part of the booths built during the Feast of Tabernacles (Neh. 8:15).
The significance of the myrtle is not revealed here, which has opened the door to a lot of guesswork on the part of commentators. Not being considered one of the stately trees of Israel but being an evergreen, it likely refers to Israel in its humble but hopeful state, at that time under Persian rule but promised restoration by the prophets, and that fits with what the angel said, “‘O Yahweh of Armies, how long will you not have mercy on Jerusalem and on the cities of Judah, against which you have had indignation these seventy years?’ (Zech. 1:12). The myrtle trees do not represent the Christian Church, although some commentators think they do.
Note that the angel did not ask “if” God would someday have mercy on Israel and restore her to glory. God had said through the prophets that he would do that, so the angel asks how long before it will happen.