Swelling and bruising are normal after surgery. It may travel to areas on your foot that weren't operated on and may go up and down.  Due to swelling after surgery, your foot may change color.  Additionally, because you haven't moved your foot as you do before surgery, the veins on your foot experience difficulty returning blood to your heart.  The discoloration should slowly go away 6-8 weeks after your surgery if you follow proper post-surgical care methods.