Final answer:
On the feast day of a martyr in the Russian Orthodox Church, a priest usually wears a dark red or purple cassock, which symbolizes life, love, warmth, and the victory of life over death.
Step-by-step explanation:
The color a priest wears on the feast day of a martyr in the Russian Orthodox Church is usually dark red or purple. This is because red is a sacred color in the church and symbolizes life, love, warmth, and the victory of life over death. It can also signify martyrdom, which is associated with the feast day of a martyr. Bright red cassocks may be worn on specific feast days, such as Sts Peter and Paul, Pascha (Easter), and Christmas.