The genome of a single person was originally sequenced, and researchers know fully that not every person's DNA sequence will be exactly the same. it is a starting point. The sequenced genome can serve as a reference, so that other people's different genes can be compared to it.
Eye color is not a single gene. If is a combination of different factors. Blue eyes is the result of a lack of pigments inside the eye. The eyes of all people have the same molecules and these molecules scatter blue light, in much the same way that the sky looks blue because the air molecules scatter blue light. The eyes of most people around the world are brown because they have the pigment eumelanin inside the eyes, which absorbs all kinds of light, including blue. Blue eyed people have a defective version of the gene(s) that is responsible for making eumelanin. Without eumelanin, the scatterd blue light is not absorbed, and can be seen by other people. Since it takes several steps to make eumelanin, and each step requires a different enzyme, it means there are several different genes that can result in blue eyes if any one of them is defective. One person with blue eyes may therefore have a different defective gene than another person with blue eyes. Blue eyes are rare among non-Europeans, but it is known and these people may have a different reason, a different defective gene, than Europeans who have blue eyes. Hazel eyed people do not have eumelanin in their eyes, but they do have a large amount of phaemelanin in their eyes. Pheomelanin looks yellow in low concentration but red in high concentration. People with no eumelanin but a small amount of phaemelanin inside the eye will have green eyes because yellow + blue = green. In contrast blue-eyed folks have neither eumelanin nor phaeomelanin inside their eyes.