CMD diagrams of the full Gaia+2MASS+WISE giants for which we will predict metallicities. We see our giant sample encompasses the upper RGB out to the AGB stellar phase. The cleanliness of the Gaia CMD is a direct consequence of the methods employed by Belokurov & Erkal (2019) in their giant selection. In the other photometric systems, the CMDs look relatively clean, and we overlay black contours spanning the 5th–50th percentile levels of the APOGEE training sample, with logarithmic spacing. It can be seen that APOGEE generally obtains metallicity estimates for the brighter red giants, with the sample being particularly deficient in blue WISE colors. How this pertains to our regression analysis is discussed later in this work.