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The proliferation of smartphones and the widespread use of camera modules necessitate complementary metal-oxide-semiconductor (CMOS) image sensors with high pixel density. The recent competitive race to miniaturize pixels has enabled commercial CMOS sensors with submicron pixels to reach sizes as small as 0.5 μ m. However, further downsizing towards subwavelength pixels faces fundamental challenges as the conventional focus-and-filter approach suffers from the diminishing focusing ability of conventional microlens arrays and optical efficiency constraints imposed by absorptive color filters. Nanophotonic color routers have emerged to overcome these challenges via efficient spatio-spectral splitting, thereby directing incident light into corresponding pixels. In particular, recent developments in free-form device optimization methods enable the design of highly efficient color routers by exploring a large combinatorial design space, which was previously considered to be intractable with conventional design methods. In this review, we comprehensively introduce a multitude of research achievements in the field of nanophotonic color routers for CMOS image sensors with a special emphasis on their design methodologies. [ABSTRACT FROM AUTHOR]