Why don’t we wear red or black?

My Sunday school teacher always tells us we can not wear red or black. My church secretary also tells us this is forbidden unless for school or work. Why?
I asked one Uncle this one time and he said because red means danger and black means death but what about the catholic priests that wear red? What about the Chrubim church that wear red? This is just confusing. If these colours are really bad for us why is it still acceptable for school uniform I should be able to tell my school it is against my religion.

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