Jerome

The Angel in white raiment signifies the glory of His triumph.

Catena Aurea by Aquinas ↗

Gregory the Dialogist

Hence Matthew also, describing the angel's appearance, says: "His appearance was like lightning, and his garments like snow." For in lightning there is the terror of fear, but in snow there is the gentleness of brightness. Since indeed almighty God is both terrible to sinners and gentle to the just, rightly the angel, witness of his resurrection, is shown both with a countenance like lightning and with garments of brightness, so that by his very appearance he might both terrify the reprobate and soothe the devout. …

Forty Gospel Homilies, Homily 21 ↗

Thomas Aquinas

And his countenance was as lightning. Here he is described by his countenance; and in this it is clear that he appeared in an assumed body. But why as lightning? Because just as lightning has brightness, so also the angels have knowledge; Dan. 10:6: and his eyes as a burning lamp. But Christ is he who enlighteneth every man that cometh into this world, John 1:9. Likewise, lightning causes terror, and so does the countenance of an angel; hence in Luke 1:9 it is said that Zacharias was frightened at the voice of the angel. …

Commentary on Matthew ↗