Colorimetry detects a color change resulted from a chemical reaction or molecular binding. Despite its widespread use in sensing, continuous monitoring of analytes with colorimetry is difficult, especially when the color-producing reaction or binding is irreversible. Here, we report on a gradient-based colorimetric sensor (GCS) to overcome this limitation. Lateral transport of analytes across a colorimetric sensor surface creates a color gradient that shifts along the transport direction over time, and GCS tracks the gradient shift and converts it into analyte concentration in real time. Using a low cost complementary metal-oxide semiconductor imager and imaging processing algorithm, we show submicrometer gradient shift tracking precision and continuous monitoring of ppb-level ozone.