I'm working on a documeint in CMYK mode
Hi @Slayer213 , InDesign doesn’t have a document color mode—page items and colors can be a mix of RGB, CMYK, or Lab color modes. It looks like your placed image and background color is RGB, and the RGB colors are outside of the CMYK gamut. You can check a placed image’s Color Space and profile assignment via Link Info, and the Color panel will tell you the mode of a fill color—the warning triangle indicates the color is out-of-gamut

When you turn on Overprint or Separation Preview the RGB color is dispayed in the Document’s assigned CMYK space—the assigned color profile and intent will affect how the color is brought into gamut:

