Effects of an external magnetic field on the energy spectrum and surface charge density of semiconductor nanoparticles (SNPs) in ionic colloids were investigated in the frame of finite element method . The maximum probability for carriers increases with the increasing of the applied magnetic field for both spherical and nonspherical nanoparticles. The effect of an applied external magnetic field on the surface charge density of SNPs is dramatically modified by the nanoparticle shape. Our observations can be well explained through the competition of the quantum confinement effects introduced by both particle geometry and magnetic field.