What is MOST LIKELY causing the discoloration of tooth #8?

The single white crown of tooth #8 contrasted against betel nut-stained teeth suggests the crown was placed AFTER betel nut staining began — the crown was shade-matched to normal teeth but now stands out as the surrounding teeth became darkly stained by betel nut. 'Poor shade matching' reflects the mismatch between the current tooth color and the crown color.