if your talking about hair colour, than fair means light, blond is the colour blond... fair can refer to a light colour of hair where blond is a specific colour. You could say; she has a fair colour of blond. (she has a light blond colour) hope this helps, let me know if you have any more questions :)

You can say : she has fair skin and blonde hair.

"Blonde" and "fair" can both refer to blonde hair. However, "fair" can also refer to skin - skin that is light and free of blemishes.