According to the Packers Blog, Tom Silverstein wrote the following about Clay Mathews and his shoes:
"LB Clay Matthews said in an interview with teammate Josh Sitton for the cable television show Rome is Burning that he had been fined $5,000 for wearing the wrong colored shoes Sunday against St. Louis.
The Packers wore their 1929 throwback jerseys that feature navy blue tops, brown helmets and tan pants. The shoes that come with the gear are dark brown.
Some players wore white shoes, but used black tape over them so that they were dark and didn't stand out.
Matthews, however, wore bright yellow shoes, which the NFL deemed as inappropriate and a violation of the league's dress code. In the interview with Sitton, Matthews read outloud the letter he received from the NFL."
Fining players for uniform violations is nothing new for the NFL. For instance, in 1986 Jim McMahon was fined $5,000 for wearing an Adidas headband during a game. The following week McMahon retorted by wearing a headband with the name "Rozelle" (after Commissioner Pete Rozelle). Rozelle stated that McMahon's retort was "a great gag".