Formation of supracolloidal chains from patchy micelles of diblock copolymers can provide valuable information to understand mechanisms and kinetics of colloidal and molecular assemblies. Direct observation of the assembled supracolloidal chains using microscopy enables statistical evaluation of the progress of the assembly, but sample preparation and data analysis processes prohibit the determination of chain length in the solution state. Here, we demonstrate supracolloidal chains induced from fluorescent diblock copolymers that can self-report the extension of chain length by a shift in emission color. Patchy micelles are engineered to perform Förster resonance energy transfer (FRET) interactions that respond spontaneously upon colloidal assembly. For this purpose, fluorophore-functionalized diblock copolymers are designed and synthesized to form patchy micelles incorporated with fluorescent dyes selectively at the patches. Then, two different patchy micelles containing FRET donors or acceptors are prepared and coassembled into supracolloidal chains. The merging of patches between adjacent micelles enables the FRET pairs to come into vicinity inside the patches, allowing the enhancement of red emission occurred by FRET upon the supracolloidal coassembly. The shift in emission color from yellow to scarlet provides macroscopic evidence that can monitor the progress of colloidal assembly simply by observing photoluminescence. Moreover, quantitative analysis of the FRET interaction can be interpreted using step-growth polymerization kinetics, permitting facile real-time tracking of chain length during the progress of colloidal assembly. Lastly, colocalization of FRET pairs within single micelles enabled direct monitoring of solution-state behavior in patchy micelles and colloidal assemblies, which could not be revealed by microscopic structural analysis.