I was thinking about the expression "black sheep" (like the odd one who doesn't fit the family norm).
. . . and that brought me to wonder if, in fact, sheep see in color, but more if they see the contrast of black to white. I've read contrasting thoughts about it. However, from the quote I have above, it makes sense and also sheds more light on the expression itself. If they tend to move 'toward' the light, I guess there's also "black sheep" within the sheep family.