The people harvested in the Wheat Harvest will be in need of clean, white robes. Initially, their robes will not be clean enough. Their robes will need to be "washed in the blood of the lamb".

Revelation 7:9-13
After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands; And cried with a loud voice, saying, Salvation to our Elohim which sitteth upon the throne, and unto the Lamb. And all the angels stood round about the throne, and about the elders and the four beasts, and fell before the throne on their faces, and worshipped יהוה [Yahweh, our god], Saying, Amen: Blessing, and glory, and wisdom, and thanksgiving, and honour, and power, and might, be unto our Elohim for ever and ever. Amen. And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they?

Revelation 7:14
And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.

Many believe that the "great tribulation" will be the "time of trouble" at the end of the world. That is possible, but doubtful. The wheat is represented by the foolish virgins. (the wise virgins represent those saved in the barley harvest). The foolish virgins lacked the faith of Jesus, resulting in stained garments. Some of these will be alive (but not ready) at the 2nd coming, but most will have already died (died, unready). Therefore, they may have gone through no tribulation on earth, at all (depending on when they lived and died).

The "tribulum" is used to separate the chaff from the wheat. The word "tribulation" is derived from the name of that tool. It is more likely that the "great tribulation" is the period of threshing of the wheat, which is part of its harvest. This will be the great tribulation. The tribulum (tribulation) will be to the wheat as the winepress will be to the grapes. For both, it will be their "time of Jacob's trouble". This tribulation / threshing will remove the impurities (chaff), leaving good, clean wheat, ready for the barn.

The people of the wheat harvest will wash their own robes in Christ's blood (Revelation 7:14). This signifies purification (cleansing), resulting in clean, white robes and their restoration. We have now seen two harvests, the barley and the wheat, each with their own robes. Yet, one remains - the grapes.