I think the gray stats just refer to skills which are important for performing the specific duties for the position the player is assigned too play. However, the gray skills might still factor into what happens on plays which aren't from the run of play. For example, My association duel today was decided a goal from my DL, smashing home the corner kick. In a case like this this, his heading and/or positioning skills may have played a role in getting onto the end of that cross; although heading is a gray stat. What's clear is the my DL was in the box, and heading is a white stat for both centerbacks and strikers so I would assume in factored in somehow in this situation.