Compression artefacts. When you have a big picture (as is recommended for colouring) and then resize it, the program has to decide how to represent the same picture with less pixels. Some file formats are better at doing this (png), some worse (jpeg). Also, if you save the picture several times over (as it happens when colouring and then lettering), the noise of these badly represented pixels becomes worse. So stay away from jpegs if you value your pictures' quality.