BALTIMORE — Royals pitcher Danny Duffy, like everyone else in baseball, wears customized cleats.

His are gray and blue New Balances. The hex codes and Pantone designations match the Royals’ official team colors. Even the number 30, a tribute to his late teammate Yordano Ventura, emblazoned on the back of each shoe fits into the color scheme.

Duffy plays within the arbitrary rules that were set in the most recent collective bargaining agreement.

But when ballplayers across the country began to receive letters from the league office claiming they violated a rule that requires “at least 51 percent of the exterior of each player’s shoes be the club’s designated primary shoe color,” Duffy spoke up.

“I think they’re just picking and choosing what they can control and what they can’t,” he said. “It’s weird.”

Indians pitcher Mike Clevinger, who often wears cleats that are tie-dyed and dotted with sunflowers, posted a picture of the warning he received on Friday on social media with the caption, “Make baseball fun again, they said, it would be fun, they said ...”

On the other end of the spectrum was former Royals player Ben Zobrist, who was threatened Saturday with a fine if he continued to wear all-black cleats with flaps “that do not comply with the uniform regulations.”

In the Royals clubhouse, starting pitcher Jakob Junis received a warning this week, too. The cleats he wore in his last start at Kauffman Stadium weren’t quite gray enough.

“I don’t even know what color his cleats were,” Duffy said. “Probably white and blue. It’s like, ‘47 percent blue is just not enough. We’re gonna have to get rid of these.’ It’s so stupid.”

The players’ frustrations stem from Major League Baseball’s efforts to make itself more appealing to younger audiences. Improving pace of play is one project commissioner Rob Manfred has undertaken to improve the fan experience.

In another instance, the players’ association has encouraged its members to use a phone application called Infield Chatter, which is intended to help players “engage with fans.”

Yet the league has also continued its campaign to rein in its players. The efforts seem contradictory.

“If your personality gets out, it’s gonna be good for MLB and it’s also gonna be good for you,” Duffy said. “MLB needs to figure out a way to trust that we’re not gonna go have middle finger emojis on our cleats. We’re gonna be responsible enough to know the limit as opposed to just flat out fining people. They want certain people to be themselves and others not.”

Duffy pointed to the Nationals’ Bryce Harper, who made waves in 2016 with his determination to “Make Baseball Fun Again.” Harper this year has sported a pair of Louis Vuitton x Supreme cleats. He’s previously worn shoes featuring the eyes of a tiger and a pair that had stars on one shoe, stripes on the other.

There’s been no public indication that Harper has faced repercussions from the league for his sartorial choices.

“MLB is actually promoting it,” Duffy said. w“MLB tweets that stuff.”

Duffy publicly expressed his annoyance on Twitter on Friday. He re-posted Clevinger’s picture and wrote, “Make the game more appealing to the youth, they said. Show more personality, they said.”

For the time being, there’s no solution. Players are stuck with the uniform policy until a new collective-bargaining agreement is ratified ahead of the 2022 season.

But Clevinger chose to resist the league’s latest nit-picking maneuver.

When he started against the Royals on Saturday at Progressive Field, Clevinger wore pink shoes that were not the pair approved by the league for Mother’s Day. The shoes, instead, were adorned with flowers and the names of women in his life, he told reporters in the Indians clubhouse.

“Ultimately, all we’re trying to do is play the game,” Duffy said. “If we like the way a certain shoe looks and it has our team colors on it and it doesn’t take away from the rest of the game — what’s the big deal?”