Who are three or four characters with blue eyes mentioned in the first six chapters?
The narrator of The Devil's Arithmetic often focuses on eye color, probably to help us imagine more clearly what the characters look like, but also as a way of establishing connections among characters.
However, to answer your question, only Aaron and Shmuel are the blue-eyed people mentioned anywhere within the first six chapters. Hannah focuses on this connection, realizing that Shmuel feels familiar to her because his blue eyes are so much like her own brother's.
Let's consider each of the first six chapters and what they have to say about eye color.
Chapter 1 doesn't mention anyone's eye color, but we're told in Chapter 2 that Aunt Eva has "deep-set coffee-colored eyes." No eye color is discussed in Chapter 3, but in Chapter 4 when Hannah enters the past, seeming to have stepped into the role of another young girl in a different family, she recalls her own family:
"She remembered her mother and her father and her brother Aaron with his big blue eyes and great smile."
Then, as she observes Shmuel, her new uncle in this strange alternate reality, Hannah sees that "he had the bluest eyes she'd ever seen, bluer even than Aaron's."
In Chapter 5, Shmuel's sister Gitl makes reference to Shmuel's eye color, too, joking about how his new wife should notice nothing but his own blue eyes when the two of them return to their home after the wedding.
When Hannah looks into the mirror in Chapter 6, she mentions that her own gray eyes are still familiar to her, that her face is the same, even though she's inexplicably become a different girl named Chaya--and her braces are gone.
Later in that chapter, Hannah meets Rachel, a girl with large green eyes.
This is all that the first six chapters reveal about whose eyes are which colors.