Question 6

When a Buddhist becomes a monk or a nun (Bhikku or Bhikkuni) and joins the Sangha or community, they give up wearing ordinary clothes as a symbol of giving up their ordinary lives to devote time to study and worship. This change of appearance is true of monks in Christianity and Hinduism too.

According to Wikipedia: "The color of modern robes varies from community to community (saffron is characteristic for southeast Asian Theravada and Mahayana groups, maroon in Tibet, gray in Korea, black in Japan)".

So there seems to be no religious significance in the choice of colour - it is just tradition.