The cuttings will start to yellow...especially at the tips, as the rooting intensifies. Rooting requires energy. There are no roots to take energy form the soil, or whatever medium, so the plant has to use stored energy, which is found in the leaf. As rooting progresses, so will the yellowing. Again, check out my journal which covers these points. You want to see the yellowing, this is the first sign that rooting is underway. Its then only a matter of days before you see the roots pop through the sides or bottom of the cube.