Dollars (主语)
are called greenbacks(谓语)
because that is the color of the backside of the paper money(原因状语从句)
两个of 意思相同,都是“.的”,都和后面的名词构成介词短语,做前面那个名词的后置定语.
the color of the backside 背面的颜色
the backside of the paper money纸币的背面
the color of the backside of the paper money纸币背面的颜色