A pochette in the breast pocket of your jacket shows that you are aware and have sense of style. This trend has been in fashion since the First World War and today its a natural element for men in all ages.
A pochette shall some how raise the colors in the rest of the outfit, either directly or as a complementary color. Match the color of your shirt, jacket, pants or socks if you want to be sure of being right. If you have a good sense of coloring, go ahead and pick a color that contrasts.