To redden is to become red, or to make something red. A face reddens when blood comes up into the skin — from heat, from running, or from feeling shy or angry. The sky can redden at sunset. A cloth can redden if the dye spreads. You can redden something on purpose, though in everyday talk people more often say “make it red” or “turn red.” “Redden” feels a little more like writing or storytelling than like a word you drop in the kitchen. Though actually, for faces, “go red” and “blush” are what most people say; “redden” sits next to those, a bit more careful, and it covers the sky and the skin as well as a shy face.

變紅、發紅。說到臉，台灣日常更常直接說「臉紅」。