Literature reports of a second shear thinning regime after shear thickening in dense colloidal dispersions are examined from the viewpoint of the material properties of the particle phase. We hypothesize that the particle modulus can limit the extent of shear thickening, and that the second shear thinning regime is a manifestation of particle softness. Model poly(methyl methacrylate) (PMMA) colloidal dispersions are synthesized and purified. Stable suspensions are shown to exhibit strong shear thinning and thickening, followed by a second shear thinning regime. Using roughened plates, we determine that the second shear thinning regime is not an artifact of slip, or adhesive failure with the rheometer tooling. These shear thickening fluids (STF)s are fabricated into composites with ballistic materials. Testing of these composites in quasistatic puncture and ballistic testing indicates shows a correlation between the suspension rheology and STF-composite performance. These results demonstrate the important role of particle hardness in dense colloidal suspension rheology.