The interaction of color was one of Albers's major preoccupations and one that he reexamined continually. Toward the end of his life, he worked on three print projects in collaboration with Gemini G.E.L. The first, White Line Squares, extended Albers's exploration of color relationships. He used the format of squares within squares, a structure that appears in a body of his work titled Homage to the Square. Each of the lithographs in White Line Square is printed in three colors. A square formed by a white line divides one of the three color areas into two parts, creating the illusion of a fourth square.