The word "unstained" refers to something that is free from any blemish or dirt. To describe the opposite of it, we can use the antonyms like "stained," "soiled," "dirty," or "tainted." These words are often used to refer to something that is contaminated or spoiled by a foreign element, leaving a permanent mark. The idea of a contrasting word to "unstained" often relies on the type of object being referred to - a shirt can be stained whereas a cake can be soiled. Therefore, it's important to pick the right antonym while describing an object and its state of cleanliness.