Robben reprimanded over long johns
German media have been speculating whether Robben's grey-coloured underwear - which the Dutchman has been sporting to protect his muscles from the freezing winter - conform to Bundesliga rules as the colour differs from the team's red kit.
"The German federation want a single colour," sporting director Christian Nerlinger told the club's website.
"We'll either have to dye them or he'll have to find another pair."
Robben, wearing his trademark longjohns, scored two goals as Bayern won the first three games after the mid-winter break to lie two points adrift of leaders Bayer Leverkusen.
"I'm an explosive player who must warm up well," he said.
"They are not beautiful but they are functional."
Wolfsburg, beaten 3-0 by Bayern earlier in the season, have failed to win in their last eight Bundesliga outings and occupying 11th spot are now looking more to the relegation zone than the title race.
Their miserable run has also seen them knocked out of the Champions League and cost coach Armin Veh his job.
Leverkusen, the league's only unbeaten team, travel to struggling VfL Bochum on Saturday and like Bayern attempt to maintain a 100 percent record since the break.
Goalkeeper Rene Adler believes the leaders, who have 44 points from 20 games, are reaping the benefits of staying at home for the break rather than following the lead of their rivals and heading off to sunnier climes.
"We worked hard during the short winter break," he told the league's website.
"We stayed here and got used to the difficult winter conditions.
"You're never going to play attractive football on these pitches but our players have the technical skills to play a decent game even on those surfaces.
"Now we're up against Bochum and it's going to be a tough task because everyone is expecting us to destroy them," said Adler.
The keeper would not be drawn on their chances of winning a first title and finally shedding their unhappy 'Neverkusen' tag.
"I've been at the club for 10 years now and experienced quite a lot," said Adler. "We don't want to get too carried away."
Third-placed Schalke 04 (41 points) can pounce on any slip-ups by the leading pair when they visit relegation-threatened Freiburg on Saturday.