B1
not mixed with other colors, substances, etc.
Dictionary examples:
I bought a plain white shirt, but my cat decided to add polka dots! (0.0)
He wanted a plain burger, but got a tower of toppings instead! (0.0)
He asked for plain coffee, but the barista gave him a cup of giggles! (0.0)
plain yogurt (16.1)
For the living room, we've chosen a plain carpet and striped curtains. (37.0)