So. In order to kill somebody with the death note, you have to know his face and name. Let's say, Bob knows both, and wants to kill John. Now he hasn't a picture of John, but he remembers his face and name. But the thing is, that when he met John, John wore contacts, that gave him blue eyes instead of his natural brown eyes.
Now he has to remember Johns face, but John has brown eyes, and he thinks in his mind, that his victim has blue eyes. So does he have to know Johns true eye color?