C. F. Keil and Franz Delitzsch on Isaiah 63:2

Isa 63:2

The seer surmises this also, and now inquires still further, whence the strange red colour of his apparel, which does not look like the purple of a king’s talar or the scarlet of a chlamys. “Whence the red on thine apparel, and thy clothes like those of a wine-presser?” מדּוּע inquires the reason and cause; למּה, in its primary sense, the object or purpose. The seer asks, “Why is there red ('âdōm, neuter, like rabh in Isa 63:7) to thine apparel?” The Lamed, which might be omitted (wherefore is thy garment red?), implies that the red was not its original colour, but something added (cf., Jer 30:12, and lâmō in Isa 26:16; Isa 53:8). This comes out still more distinctly in the second half of the question: “and (why are) thy clothes like those of one who treads (wine) in the wine-press” (begath with a pausal á not lengthened, like baz in Isa 8:1), i.e., saturated and stained as if with the juice of purple grapes?