Update: Apparently, a real fix for this is on the way.
Acknowledgements: thanks to elios and bochoven on MacAdmins Slack for figuring out what was going on.
If the icons in your Munki repo looked fine on your 10.12 and 10.11 clients, and then a few of them suddenly look sort of faded (for example, Word and Excel in this screenshot) in 10.13 clients, it's apparently because of a change in the way 10.13's Safari webkit displays .png files missing the ColorSync profile in the Get Info context menu (you'll still see the ColorSync profile if you open the .png with the ColorSync Utility).
The simple fix is to do the following:
- Mount the Munki repo share using a Mac running macOS 10.13.
- Delete the offending icons from /PATH/TO/MUNKI/REPO/icons/
- Regenerate new icons with /usr/local/munki/iconimporter /PATH/TO/MUNKI/REPO
Note: Icons generated using MunkiAdmin or sips will be fine, too, even if generated using a machine running macOS 10.12.