greyscale with colored background
An artwork where the subject is in greyscale but the background is colored, it's usually also a simple background.
Do not tag these as greyscale, partially colored or spot color.
However if there're parts of the image other than the background is colored, such as the subject's eyes, spot color can be used. If the subject has enough coloring on itself that you'd tag it partially colored, simply use that tag instead and do not tag it as greyscale with colored background.
Examples
