A.T. Robertson Commentary Revelation 6:11

"And there was given them to each one a white robe; and it was said unto them, that they should rest yet for a little time, until their fellow-servants also and their brethren, who should be killed even as they were, should have fulfilled [their course]." — Revelation 6:11 (ASV)

A white robe (στολη λευκη). Old word from στελλω, to equip, an equipment in clothes, a flowing robe . For the white robe for martyrs see 3:4f.; 4:4; 7:9,13; 19:14.

That they should rest (ινα αναπαυσοντα). Sub-final clause with ινα and the future indicative (6:4) middle rather than the aorist middle subjunctive αναπαυσωντα of Aleph C.

Yet for a little time (ετ χρονον μικρον). Accusative of extension of time as in 20:3. Perhaps rest from their cry for vengeance and also rest in peace (14:13). For the verb αναπαυω see on Mt 11:28.

Until should be fulfilled (εως πληρωθωσιν). Future indefinite temporal clause with εως and the first aorist passive subjunctive of πληροω, to fill full (Matthew 23:32; Colossians 2:10), "until be filled full" (the number of), regular Greek idiom.

Which should be killed (ο μελλοντες αποκτεννεσθα). Regular construction of articular present active participle of μελλω (about to be, going to be) with the present passive infinitive of αποκτεννω, Aeolic and late form for αποκτεινω, to kill . John foresees more persecution coming (2:10; 3:10).