Although color and appearance have a psychological component, they are material properties that can be of considerable significance and economic importance to designers. Paint systems are materials that have a primary role of modifying the appearance of an engineering structure. The determination of structure-property relationships is recognized as a defining feature for different branches of materials science. These relationships are at the heart of the predictive models required for the development of new materials. Here we explore some of the microstructural parameters that control the appearance of platelet-containing paint systems. The orientation of platelets is a microstructural property known to affect the appearance of platelet-containing paints. However, prior works examined only a limited set of samples. Additionally, they did not identify other potential microstructural/formulation changes that could affect the relationship between orientation and appearance. In this paper, the orientation of the platelets was verified to be the major microstructural parameter that influenced the lightness of the paint system. In addition, the strength of this influence was shown to vary because of specific formulation changes, such as platelet size and volume fraction. The relationship between orientation and appearance was shown to depend on an additional microstructural property that is related to the size of the gaps between the platelets. We termed this additional microstructural property the gap factor. The size of the gaps between the platelets, and the coverage/shadowing of platelets by other platelets, was found to affect the scattering behavior of the system, particularly near the specular reflection direction. For example, larger gaps increase the probability of multiple reflections between the platelets. These results will guide the on-going work to develop improved structure-property relationships. Improved structure-property relationships are expected to provide more accurate models for the appearance of platelet-containing paint systems.