One example is hair color. There is one gene that controls the color pigment of your hair, while the other controls whether or not your hair will have pigment. The first gene is decides whether you're blonde, brunette, etc. The second gene controls pigment distribution. If it's turned on, your hair will have the pigment that was decided by the other gene. If it's off, your hair won't have pigment (or in other words, you'd be albino).
I hope this helps!