@Laotionne - Some people think that wearing black running pants and black shirts when they are working out helps burn more calories. The thinking behind this is that the black will absorb more light and create more heat, so more calories will be burned.
This is one of the reasons you see some people working out and wearing black plastic trash bags. I don't think the trash bag shirt is as popular as it once was, but that's why the trend got started. People wanted to burn more calories.
Regardless of how this started, wearing black does not help you burn more calories. This may make you sweat more, so you will lose more water weight, but that is short term, and all the sweating can also lead to dehydration, which can create some serious problems.