Re: I am to transfer the colours in my document from RGB to CMYK. I have already tried googling it, but none of the solutions were succesful. I have yet tried changing the photos in a different editing-program. Hoping for an easier way. Thank you.

We probably need more information, but if the goal is to prepare files for print you can leave your colors in RGB in the .indd file and convert to the correct CMYK profile (which should be provided by the printer) when you export to PDF.