engrisailler

A rare, literary verb built from grisaille (grayness/dullness): to turn something gray or dull, or figuratively to cloud over — a mood, a memory, a youthful enthusiasm losing its color. You'll only meet it in literary prose.

• Les années avaient engrisaillé son regard autrefois si vif. — The years had dulled his once-lively gaze.
• Le ciel s'engrisaille à mesure que l'hiver approche. — The sky turns increasingly gray as winter approaches.
• Le doute engrisaillait peu à peu sa foi d'autrefois. — Doubt was gradually clouding over his former faith.

Rare and literary enough that there's no genuine common-mistake pattern — it's built transparently from grisaille (grayness), which makes its meaning guessable even if you've never seen the verb itself.