The phrase "turns yellow" refers to the process of something changing its color from its original hue to a yellowish tone. There are several synonyms that can be used to describe this phenomenon, such as "becomes yellow", "takes on a yellow tint", "fades to yellow", "turns golden", "yellows", "pales to yellow" and "tinges with yellow". These synonyms can add variation to your writing and help paint a more vivid picture of what you are describing. Whether it be a flower, a paint color, or even a person's teeth, using different synonyms for "turns yellow" can give readers a more interesting and detailed insight into what you're describing.