Sister chromosome exchanges are recombination events that occur between homologous chromosomes and are often the result of:

Sister chromosome exchanges are recombination events that occur between homologous chromosomes and are often the result of exposure of the cell to DNA damaging agents. 
Cross over, or recombination, is the exchange of chromosome segments between non sister chromatids in meiosis. Crossing over creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic diversity. 

The best way to tell its true color is to look at the minerals streak