be, look, seem

extremely, fairly, very, etc.
The food was fairly plain, but well cooked.

completely
The dress was completely plain, but quite stunning.

plain blue, white, etc.
a plain white shirt

be, seem
become
make sth
They made it plain that they were against the idea.

fairly, very, etc.
absolutely, perfectly, quite
Within weeks, it became perfectly plain that we were in the grip of a tyrant.

increasingly
reasonably (esp. BrE)

to
It is all very plain to me.

Plain is used with these nouns:
carpet, chocolate, clothes, common sense, cotton, crisp, dress, English, envelope, fabric, face, fact, fare, flour, folk, food, fun, glass, language, linen, luck, paper, prose, shirt, stupidity, T-shirt, text, tile, truth, white, woman, word, wrapper, yogurt