"My favorite store."
Translation:Mi tienda favorita.
Typically, you use "de" when the following word is a noun (una ensalada [de] queso/A salad made of cheese), and leave out the "de" if the following word is an adjective (Un abrigo rojo/A red coat). Saying "Un abrigo de rojo" would be saying "A jacket made of red," but red isn't really an actual substance or material you can make things out of, just a word to describe an object.
Hopefully this helped out!