 If the shadow mask or aperture grille becomes magnetized, its magnetic field alters the paths of the electron beams.This causes errors of "color purity" as the electrons no longer follow only their intended paths, and some will hit some phosphors of colors other than the one intended.For example, some electrons from the red beam may hit blue or green phosphors, imposing a magenta or yellow tint to parts of the image that are supposed to be pure red.