The phrase "were plain" can be replaced by many different synonyms depending on the context. Some similar words that could be used in its place include: were unadorned, were unembellished, were unornamented, were undecorated, were simple, were basic, were uncomplicated, were straightforward, were mere, were ordinary, were common, were unremarkable, were unexceptional, were pedestrian, were humdrum. Each of these terms conveys the idea of something being unadorned and lacking in decorative elements. Using a synonym can add variety to writing and help avoid repetition.