writing in red; red ink writing
Refers to the act of writing in red ink or the resulting red text, often used for emphasis, corrections, or official annotations.
The teacher wrote comments on the exam paper in red ink.
Compound of 朱 (しゅ, 'vermilion, red') and 書き (かき, 'writing'), the continuative form of 書く (かく, 'to write').