Two print paths compared: through the CUPS filter chain the 4x6 photo job reaches the printer as grayscale raster; a direct IPP Print-Job carrying the JPEG lets the printer firmware render it in color.
Linux

A CUPS driverless queue prints 4×6 photos in grayscale whatever the options — a direct IPP Print-Job prints them in color

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Every color option on a CUPS driverless queue still gave grayscale 4×6 photos on a Canon inkjet. A direct IPP Print-Job with the JPEG prints in color.