The color of your Trainer Card will change each time you complete one of the following feats. The order of colors is as follows (challenges can be completed in any order): Blue, Bronze, Silver, Gold, Black.
Deafeat the Elite Four
Obtain all Entralink Powers
Obtain all Pokemon Musical Items
Complete the National Pokedex
Get a 49 Streak in both Super Single and Super Double Subway lines in the Battle Subway (Good Luck)