Advance in wet chemistry enables the sophisticated design of nanocrystal quantum dots (QDs) and allows unprecedented color purity and brightness, promising their useful applications in a variety of light-emitting applications. A representative example is core/shell heterostructures, in which charge carriers are effectively decoupled from structural artifacts to generate photons efficiently. Despite the development of widely accepted synthetic protocols for Cd- or Pb-based QDs, the progress in heterostructuring environmentally benign QDs has been lagging behind, and so is the practical use of these QDs. Herein, we present a design principle for InP/ZnSexSi1-x heterostructured QDs. A principal design approach is the growth of uniformly thick inorganic shell consisting of a ZnSexS1-x inner shell and a ZnS outermost shell that effectively confines electrons from spreading inward of QDs. Comprehensive studies across synthesis, spectroscopic analysis, and calculation uncover that the presence of Se near the InP emissive core enables a uniform shell growth to an extended thickness and the S-rich exterior shell ensures the decoupling of the electron wave function from the surface trap states. Engineering composition profile across multiple shells enables us to realize InP/thick-shell QDs meeting the requirements of light-emitting applications such as high photoluminescence quantum yield, narrow spectral bandwidth, and enhanced photochemical robustness. We capitalize on bright, robust, and color-pure InP/ZnSexS1-x/ZnS QDs with a range of emission wavelength covering from cyan to red regions by exemplifying their use in the primary-color light-emitting diodes (peak external quantum efficiency of 3.78 and 3.92% for green- and red-emitting ones, respectively).