That looks normal to me. Bruising, especially internal can take days even weeks in some cases (more traumatic injury, not a neuter) to come to the surface. One of my dogs was actually much worse then that. Some dogs show very little bruising or swelling, some show some, some show a lot.. all depends on the individual dog. Think of the colours you see when you get a bad bruise, it goes red, purple, green, yellow, and eventually fades.
I wouldn't worry but I'm sure your vet wouldn't mind if you stopped by for some one to take a quick look and re-assure you. I would call and let them know you're going to stop by. Even one of the vet techs might be able to take a quick peak if a vet isn't available (although a vet would likely prefer to take a look in case of any liability issues).
But bottom line.. I wouldn't worry
