* "Fair" as in "a fair price" or "a fair deal" is juste.
* "Fair" as in "a fairground" is foire.
* "Fair" as in "light-haired" is blond (for males) or blonde (for females).
* "Fair" as in "beautiful" or "pretty" is beau (for males) or belle (for females).
So, to know the correct French spelling, you need to provide more context!