We made structure analyses of colloidal crystals in salt-free silica dispersions with water as dispersant by Kossel line analysis and estimated bulk moduli of static elasticity. Due to the density difference between silica particles and water, there were observed sedimentation effects and small but still detectable gravitational pressure effects on the colloidal silica crystals. Using laser diffraction by a parallel Ar laser beam, we observed intrinsic Kossel images of colloidal crystals to accurately probe symmetry structure. We photographed the Kossel images of colloidal crystals systematically at different heights and found that the crystals keeping cubic structure received uniform compression due to gravity. High precision measurement of a lattice constant as a function of height enabled us to estimate the elastic moduli of the colloidal crystals.