An idiom meaning one's expression (especially the eyes) changes—eyes light up, become fierce, or show excitement/anger; figuratively, to become suddenly serious or zealous about something.
あの選手は賞金の話を聞いて目の色を変えた。
That athlete's eyes lit up when he heard about the prize money.
彼女は不正の証拠を見つけると目の色を変えて調査を始めた。
When she found evidence of wrongdoing, she changed her expression and began investigating intensely.
子どもたちはお菓子の箱を見ると目の色を変えて集まった。
The children gathered eagerly, their eyes lighting up when they saw the box of sweets.