Rasterizers are used to convert the document's content into a dot raster with some kind of colour format for each dot. Each rasterizer might support all or only a subset of the available colour formats. Think about a CUPS raster as a dot raster which has exactly one colour format: the one defined when it was created. A PDF rasterizer might support all colour formats. Or at least a large subset of it. Always expect the colour format you need for your printer isn't available. You might need a BGR format while only an RGB is provided. Or you might need a grey scale format, while only a monochrome raster is available. So be prepared and select a different one if possible. Else reject the job.

libcapsdriver at least emulates the monochrome colour formats if the rasterizer itself doesn't support it, and a printer driver requests for it. More emulations/converters might follow.

Ask caps_drv_job_limits_get() for the available colour formats. You need to check it at least once per document, because each time a different rasterizer with different colour format support could be in use.