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Colorimetry is a powerful sensing principle that detects a target analyte based on a reaction-induced color change. The approach can be highly sensitive and selective when a sensing material that reacts specifically with the analyte is found, but the specific reaction is usually accompanied by slow recovery and irreversibility, making continuous monitoring of air quality difficult. Consequently, colorimetry is often one-time only and single-point measurement. To overcome the difficulty, the present work reports a combined microfluidic and colorimetric approach that measures time evolution of a color gradient along a microfluidic channel via a complementary metal-oxide-semiconductor (CMOS) imager. The change of the color gradient provides continuous monitoring of the analyte concentration over many hours, and the principle and capability of the approach is demonstrated by theoretical simulation, and experimental validation with real samples.