The right way to work with images (rgb or cmyk)

A better place to pose such questions might be the InDesign forum. 

That having been said, our recommendation at Adobe is to keep your assets in the original color space as long as possible in the workflow.

Let's assume you are dealing with digital photos placed into an InDesign document. Typically, digital photographic images are provided as either highest quality JPEG or compressed, lossless TIFF in their original RGB-based workspace with an ICC color profile, typically but not limited to either sRGB or Adobe RGB.

We recommend that you place those images, without any downsampling or color space conversion, directly into your InDesign document. Export to PDF using the PDF/X-4 preset, choosing the CMYK output intent profile appropriate to your final printing situation, but don't, repeat don't, repeat yet again don't convert any colors. That's the job of the RIP or DFE of your printer. Combined with the ICC color profiles in your PDF file and the capabilities of any reasonably modern RIP (i.e., anything released or updated within the last ten years), the proper color conversions will be done automatically for you and probably much better than you can do yourself!

This type of workflow has the advantage of providing a much higher degree of device independence allowing your PDF file to be printed on a wide variety of differing devices and yielding the best and consistent  output in all cases.

(Note that unfortunately there are many printers and prepress operators who simply aren't aware of modern PDF workflows or even the capabilities of the prepress systems and print equipment they are using. In fact, creating and submitting PDF/X-1a – all CMYK with flattened transparency – in lieu of color-managed content with live transparency to most results in lower quality printed output!)

          - Dov