Pigment migration occurs when one ink migrates into another ink that is printed over the other. It’s like the green ink migrating into the white ink on the same design.
However, dye migration occurs when the dyes of the garment migrate up into the ink that is printed on it. It simply is the pinkish shade you see on a white print over a 50/50 red t-shirt. Dye migration is also known as bleeding.