Starbucks is updating its uniform policy, emphasizing the appearance of its recognizable “green aprons” as part of CEO Brian Niccol’s broader plans to revive the slumping brand.

Beginning May 12, baristas at stores in North America will have to wear solid black tops under their aprons, a change from employees being allowed to wear any colored shirt, in an attempt to “allow our iconic green apron to shine and create a sense of familiarity for our customers,” Starbucks said in a release.

The company is also tweaking pant colors, with employees soon to be required to wear khaki, black or blue denim, a change from being allowed to wear navy, gray or brown.

The coffee chain has been looking for ways to bring customers back into stores. Starbucks’ sales have slipped for four straight quarters, the longest decline in years. Some customers stopped going to Starbucks because of high prices for drinks and long wait times, and hundreds of its stores have voted to unionize to push for better pay, benefits and working conditions.

Starbucks said the dress code changes will help it “deliver a more consistent coffeehouse experience that will also bring simpler and clearer guidance to our partners, which means they can focus on what matters most, crafting great beverages and fostering connections with customers.”

Workers United, a union representing some Starbucks employees, criticized the change, saying in a statement that the company should focus on collective bargaining instead.

“Starbucks’ top priority should be finalizing fair contracts with union baristas so we have the staffing, guaranteed hours, and support we need to do our jobs. Workers United has written to Starbucks demanding no dress code change be implemented at union-represented stores until bargaining concludes,” said Jasmine Leli, a barista and an union bargaining delegate.

Other changes

It’s just the latest change that Niccol has initiated at Starbucks. Earlier this year, the chain eliminated a significant number of drink and food options as part of an overhaul to cut 30% of its menu to help reduce wait times. It also axed 1,000 corporate positions.

He’s also reimagined the vibe inside Starbucks’ locations by instituting a new policy restricting restrooms to paying customers. Plus, it tweaked its name to “Starbucks Coffee Company” to reinforce its coffee roots.

Niccol has also brought back a company tradition of baristas doodling on cups, as well as self-serve milk and sugar stations.

Niccol admitted in the company’s most recent earnings call that there’s “still room for improvement,” but he’s confident that Starbucks is “on the right track.” Since he took the helm last September, Niccol has rolled out a series of changes that have received a “positive response” from customers.

Starbucks will give a glimpse if these changes are working in its next earnings report in the coming weeks.