ほお紅や口紅を差すだけで表情や笑顔がどんどん変わってくるんです。

。

Just by applying some rouge or lipstick, your expression and smile change more and more.

Literal

Rouge [and-や] lipstick [object-を] apply [only-だけ] [with-で] expression [and-や] smile [subject-が] more-and-more change-come [explanatory-んです].

Beauty/makeup advice. 差す ('to point, insert, apply') is used for applying lipstick, rouge, and eye makeup — a specific usage different from 塗る ('paint on'). どんどん ('more and more, rapidly, steadily') is a 擬態語 for progressive, accumulating change. 変わってくる uses ~てくる for change approaching the speaker's view. や joins nouns with 'X and Y and so on' (non-exhaustive).