NFHS 3-5-6: "Undershirts shall be a
single solid color similar to the torso of the jersey and shall be hemmed and not have frayed or ragged edges. If the undershirt has sleeves, they shall be the same length. Only one visible logo is permitted. See Rule 3-6 for logo requirements." [emphasis added]
It appears manufacturers are now making undershirts using thread that differs from the color of the fabric, which naturally, is sparking debate among officials as to the legality of the undershirt. One state (I forget which) considers all three undershirts in the photo to be legal if worn under a black jersey.
Absent any specific rules book or case book guidance regarding thread color, I would consider all three pictured below as simply black.
What say you?
Mike