Sonic Pigment is a web-based, interactive spatial audio mixing tool that treats sound as a transient, tactile visual medium. Traditional Digital Audio Workstations (DAWs) often abstract the mixing process into sterile knobs and numerical panning sliders. This project reimagines spatial mixing by allowing users to upload multi-track audio stems and spatially position them by "painting" on a 2D canvas, which is mathematically mapped to a 3D binaural audio space utilizing Head-Related Transfer Functions (HRTF). The tool features generative, audio-reactive brush tips that pulse with the amplitude of the music, fading particle trails that represent the ephemeral nature of sound, and a direct-manipulation interface that bridges the gap between visual generative art and functional audio engineering.