Computed axial lithography, when used in polymeric systems, directly solidifies freeform three-dimensional geometries inside liquid or gelled materials. Currently, this patterning system operates in open loop where projections are designed prior to the print so identification of errors and corrections can only be done after the printed object has been processed. This work introduces an in-situ 3D refractive index monitoring system to track localized material conversion by performing tomographic reconstruction from color Schlieren images. Our system successfully reconstructed evolving phase objects inside resins and the reconstruction quality was verified by comparison with isosurface laser scans. The technique provides support for physics-based real-time pattern modification to improve print fidelity and reduce manual iteration time when experimenting with new materials.