[SanDiegoSteve]

Yankee Stadium uses Universal Ground Rules as determined by MLB. It has no special local ground rules published.

The top of the outfield fence is blue, but I believe for the purposes of the rule, that the yellow line rule applies, which states that all yellow lines are in play unless the ball goes past them.

The closest other ground rules that could apply are that the facings of railings surrounding the dugouts and photographers' areas are in play, and batted or thrown balls resting on the rotating signage behind home plate or along first base or third base stands are in play. Perhaps it's the same for a ball that rests on top of the fence too.