Rosario, who wore a well-received ensemble featuring a silver chest protector and shin guards with the road gray uniform in Houston in the season-opening series, wore a chest protector heavily decorated with white features at Double-A Tulsa last year.
"I don't know why," Rosario said. "They told me I could wear white in the Minors but not in the Majors. It's OK. I still get to wear my purple mask. The fans like that."
An MLB spokesman said the new Basic Agreement includes the following in its uniform regulations: "Catchers' shin guards and chest protectors may not contain any white (other than the corporate logo)."
The spokesman said it's a safety concern. A white ball against the background of white catcher's gear poses challenges, such as for fielders seeing the ball off the bat.
Uniform regulations are administered by MLB's on-field operations department, led by Joe Garagiola Jr.