After replacing my old color laser, I came across an issue where no matter what, I couldn't print in color. The printing software was set to use color, local CUPS was set to color, the CUPS server was set to color. Even if added as a local printer to my workstation's CUPS, the color was still MIA.
I learned that CUPS has hidden secondary setting for colors, that is not exposed to its web GUI. The actual master switch for color or monochrome is hidden away under "other options" in CUPS' graphical GUI. If you're administrating a CUPS server without access to the graphical GUI, the setting is in /etc/cups/printers.conf, as Option printer-color-mode. It defaults to monochrome.
The reason it defaults to color appears to be a bug report against CUPS by a large school's administrator, who complained that the students waste so much color because of the default. I can only assume this change also created the huge confusion for us, who have used CUPS since forever, but have never encountered this kind of a hidden default master switch value.
To fix this, you have two options: through a GUI that reveals the "Other Options" or just editing the printers.conf directly.
The GUI in many GTK based things appears to be a fast way, find it in your settings dialog or just run system-config-printer from a terminal.
The setting is well hidden under Job Options, towards the bottom under "more" and Other Options, as print-color-mode. Flip that to "color" and save.
Other option is to stop the CUPS daemon and editing /etc/cups/printers.conf.
Look for "Option printer-color-mode monochrome" and change "monochrome" to "color", then start the CUPS daemon.
The world is no longer in greyscale.