When Brenda compliments Monica on her top, and Monica replies "You're not getting it", the way she scrunches up her top reveals a portion of her bra (and strap) and it's white; neither pink, nor lacy. However, the bra strap that Rachel tucks back into Monica's top later is pink, matching Brenda's pink bra which all the fuss was about.
Monica thinks the maid has stolen and is wearing her pink bra and convinces Chandler to sneak a peek. But when Chandler agrees, he says to Monica, "Okay, but if it's not her bra, can you just let the woman clean the apartment?" He should have said, "If it's not *your* bra."