I think Eliza has it right - here's the FIFA regulation about underwear, etc:

27 Items worn under Playing Equipment

27.1 Players may use Equipment items worn under the Playing Equipment,
such as underwear, T-shirts and thermal shorts. Such Equipment items worn
under the Playing Equipment may be separate from, or permanently attached
to, the shirt or shorts. The Colour of any such visible Equipment items worn
under the Playing Equipment must be of the same group of a basic Colour as
the corresponding part (e.g. sleeves or legs) of a Playing Equipment item. In the
event that a Player is wearing a short-sleeve shirt as a Playing Equipment item,
a long-sleeve T-shirt worn under the Playing Equipment must be of the same
visual appearance as the long-sleeve shirt worn by other Players of the same
team.

LFF