What is a word or phrase that expresses the idea of purposely making something seem better by comparison? For example, lemon Starburst was created to make the cherry flavor seem that much tastier.
The name for something that is used to make something else look different by comparison is a foil:
foil, noun. Anything that serves by contrast of colour or quality to adorn another thing or set it off to advantage.
The presence of a foil induces a cognitive bias known as a contrast effect.
I’m not aware of any terms that mean to create a foil or to induce a contrast effect.