More than 1,000 Starbucks baristas at 75 U.S. stores have gone on strike since Sunday to protest a new company dress code, a union representing the coffee giant's workers said Wednesday.

Starbucks put new limits starting Monday on what its baristas can wear under their green aprons. The dress code requires employees at company-operated and licensed stores in the U.S. and Canada to wear a solid black shirt and khaki, black or blue denim bottoms.

Under the previous dress code, baristas could wear a broader range of dark colors and patterned shirts. Starbucks said the new rules would make its green aprons stand out and create a sense of familiarity for customers as it tries to establish a warmer, more welcoming feeling in its stores.

But Starbucks Workers United, the union that represents workers at 570 of Starbucks' 10,000 company-owned U.S. stores, said the dress code should be subject to collective bargaining.

"Starbucks has lost its way. Instead of listening to baristas who make the Starbucks experience what it is, they are focused on all the wrong things, like implementing a restrictive new dress code," said Paige Summers, a Starbucks shift supervisor from Hanover, Maryland. "Customers don't care what color our clothes are when they're waiting 30 minutes for a latte."

Summers and others also criticized the company for selling styles of Starbucks-branded clothing that employees no longer are allowed to wear to work on an internal website. Starbucks said it would give two free black T-shirts to each employee when it announced the new dress code.

Starbucks said Wednesday that the strike was having a limited impact on its 10,000 company-operated U.S. stores.

"Thousands of Starbucks partners came to work this week ready to serve their customers and communities," the company said in a statement. "It would be more productive if the union would put the same effort into coming back to the table to finalize a reasonable contract."

Starbucks Workers United has been unionizing U.S. stores since 2021. Starbucks and the union have yet to reach a contract agreement, despite agreeing to return to the bargaining table in February 2024.

The union said this week that it filed a complaint with the National Labor Relations Board alleging Starbucks' failure to bargain over the new dress code.