Sporting a fiery shade for a confidence boost could make you feel stronger, I know that works for me. 

Red is energizing to wear and we tend to tie it in with aggression, which could make your next work out more powerful.  For instance, when someone gets angry, they see 'red'.  Studies show that red can have a physical effect, increasing the rate of respiration and raising blood pressure.  Consider red a great excuse to go shopping for a motivational work out outfit!