Dispersions of monodisperse macroionic spherical particles self assemble into bcc or fee crystalline colloidal arrays (CCA), which efficiently Bragg diffract light in the near-IR, visible, and UV spectral regions, depending on the lattice constant and the crystal structure. We report here the observation of an anomalously intense "secondary diffraction", which occurs at half the wavelength (lambda(B)/2) of light diffracted by the lattice planes with highest particle density (the bcc (110) or the fee (111) planes). This diffraction is > 10-fold more intense than the primary diffraction at lambda(B), and is narrow for light incident normal to the bcc (110) or the fee (111) planes. This secondary diffraction results from fortuitously superimposed diffraction from numerous lattice planes oriented such that they diffract ca. lambda(B)/2 light. We quantitatively modeled the primary and secondary diffraction by using dynamical diffraction theory. This efficient diffraction phenomenon should be useful for optical device fabrication.