The representation of skulls in the fabric patches used by the bike riders for their vests and jackets symbolizes their commitment to the club. It is actually a representation of loyalty towards their passion, riding community, and their fellow men. Similarly, bikers are seen to be wearing illustrations of pirates and cross bones that also symbolizes their loyalty till death. 
Many of the clubs have been observed to be having their specific designs for such insignia, patches, and badges. The color present in these designs represents the geographical territory of their gang or club.