Safari 12's pinned tabs now show garish, distracting, and unpredictable colored icons in place of the subdued grayscale icons of previous versions. Is there a way to decolorize these icons?
The app Faviconographer hacked favicons onto Safari tab prior to the feature officially rolling out.
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.