Do you need flux? And, if so, which one?
No, you don’t have to use flux. Your first coat of enamel can be flux, white, or any other colour/colours of your choice. Reds and pinks react with the metal, so don’t choose them for a first coat.
Tutors often show beginners how to apply the first coat using flux because it is simple and a whole class can learn sifting or wet-laying together without being confused by possible alternatives.
Enamelling flux serves a number of purposes.