Narrator Daniel describes the appearance of himself and his sister Erika very early on in the book and during much happier times.
On page 4, he states that one of his photos in his photo album shows him as a six-year-old "with pudgy cheeks and thick black wavy hair." In another picture on page 16, he describes his sister, two years younger than him, as a "little mouse."
She had brown wavy hair and brown eyes and a little nose that she would rub when she was worried.
Since he has black hair, we could probably assume Daniel has brown eyes like his sister.
The only other time Daniel describes either of their appearances is at the concentration camp on page 63 when he mentions Erika is "skin and bones." This suggests that their appearances are too hard to talk about. Even before they were sent to Lodz, he may have found it difficult to talk about what he and his family looked like while they their appearances, as well as culture, were being severely parodied and criticized.