Is there a way to decolorize Safari 12 pinned tab icons?
The app Faviconographer hacked favicons onto Safari tab prior to the feature officially rolling out. 

https://timingapp.com/faviconographer/

It would likely be fairly straightforward to modify the app to re-grayscale the icons.


  If you have a legitimate interest in the source code, contact me.


Try asking the dev for the feature, or for the source code and build it yourself.