A method of manufacturing a color display tube having a magnetic quadrupole post-focusing mask, in which the mask is magnetized by magnetic field shaping means of soft iron, or the magnetic equivalent, having raised portions, for example strips placed against opposite sides of the mask sheet and subjected to an external strong magnetic field by an electromagnet to induce magnetic poles through the thickness of the mask sheet. The geometric arrangement of the raised portions of the field shaping means is such that cyclically a north pole, a south pole, a north pole and a south pole are induced into the sheet material surrounding each aperture. Using this technique, the entire area of a mask sheet can be magnetized in one operation. Also masks for small tubes and high definition tubes can be made.