NFHS 3.2.7
"Exposed undergarments, if worn, are considered part of the official uniform. All exposed undergarments shall be black, white, gray or a solid school color."
I think that's the passage relevant to the test question.
Art. 7 goes on:
a. Exposed upper-body undergarments do not have to be the same color as exposed lower-body undergarments.
b. All exposed upper-body undergarments shall be the same color, and all exposed lower-body undergarments shall be the same color.
Not included in the NFHS book that I could find was if'n the umpires had any exposed undergarments if they had to be the same color.
edit: except for t-shirts, if worn.
Ted