1. Color Swatches
If it was my project, I would update my color swatches to grayscale and the style will be updated as well. Or vice versa.
Of course, it's something easier to do if you plan from the start that you'll use 2 versions; this way you add color swatches for your grayscale 60% and name them this way, and then replace that recipe with the color one you want. Or vice versa. I hope I make sense.
2. Pictures: Move them
If you have a lot of pictures to replace, you can always move the color pictures somewhere else and put the grayscale in the linked folder, using the same names.
Then you go in the Links panel and simply relink one image that is now in the linked folder, and it will link back all the others as well from the same folder with the same name... They will get updated in InDesign, at the same exact size and position.
It's really the fastest way to update everything. If you have a lot of pictures to update, I don't know exactly if it works for InDesign but in Quark, I even put my mouse on the numeral keypad Enter Key and its weight presses the "ok" button to update, so I don't even need to click anything if there's a lot of images to update :D (I know it's ultra lazy, but admit it, it's clever!)
3. 2x Documents
Personally, I prefer to prepare 2 documents for this; one grayscale and one in color. If there's lot of changes, I simply use the grayscale version as main one and then do a duplicate in color with the tricks above. My priority will always be on the version that goes on press and I guess it's the grayscale one for you.
4. Why not use the "insert" command in Acrobat Pro
You can still do what I mentioned above, export your 5 pages from InDesign and then use the "insert" command in Acrobat Pro to replace the grayscale pages with the color ones. Or Vice Versa.
That seems faster than trying to find "automatic" solutions and at least you can verify manually if you replaced the right pages.
- Printers usually ignore registration marks when there's obviously
only these using CMY, just so you know. You simply need to mention it's a one color job and you'll be fine. When they "rip" your work, they don't send 4 separations for no reasons, plates are not free. And real proofs are done with the exact color separations too.
- And there's no color separation on digital printing, it's like
printing grayscale on your home inkjet.
- InDesign colorbars are decorative to printers as well, they use their
- With Acrobat Pro you can always flatten all this to
grayscale in Preflight if you want to be certain there's no CMY left
and it can even convert your whole PDF to grayscale, if that can
help. I still suggest you do your color changes in InDesign and adjust your grayscale pictures in Photoshop for better results.
- One last thing, if that project is meant to be printed, know that you
could always provide your 5 color pages as a separate document and
mention to your printer where they go. As Jooja mentioned, these will
be printed separately anyway and need to be 'imposed'. In fact the
printer himself will isolate them to print them separately...! So
maybe you could ask the printer if you can simply provide a 5 pages
PDF with your color pages and add a note on the grayscale pages in
the PDF to mention the color page goes there.