the *Euclid* survey, to map the color distribution of galaxies to the *Euclid* depth ($i$$\sim$24.5 AB). We used the SOM algorithm (a manifold learning technique for nonlinear dimensionality reduction) to generate a topologically ordered 2D representation of the high-dimensional color distribution[^1]. Galaxies from COSMOS were then matched back to the self-organized map according to their best-matching color cell in the SOM. This sorting of galaxies enables a variety of analyses, including the density of galaxies in different parts of color space, the median 30-band photometric redshifts from COSMOS as a function of position in color space, and the distribution of spectroscopic redshifts on the map (Figure \[figure:colors\]). Importantly, by placing all existing spectroscopy from the COSMOS field on the map, we reveal regions of color space for which no galaxies have existing high-confidence redshifts. Of greatest importance for the C3R2 survey are: (1) the current spectroscopic sampling across color space, and (2) the source density as a function of position in color space, as more common galaxies will contribute more to the cosmic shear signal.

Existing Spectroscopy Across Galaxy Color Space
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For C3R2 we need to identify the regions of galaxy color space for which spectroscopic redshifts already exist 