I managed to fix the icon on my computer. I won't share any of the files, but I will describe my process for those who are also bothered by the unnecessary use of color in the menu bar that only has monochromatic icons.
1. I used this app
https://github.com/insidegui/AssetCatalogTinkerer to open the Assets.car file located in /Applications/Parallels Toolbox.app/Contents/Resources/. I extracted all the assets and edited the StatusBarIcon.png and
StatusBarIcon@2x.png in Photoshop to remove the red bars.
2. I made a new Cocoa Application project in Xcode and added all the assets including the modified ones to the Assets.xcassets folder inside the project.
3. I compiled the application, extracted the Assets.car file and replaced the old Assets.car inside the Parallels Toolbox Resources folder.
4. After restarting the app I now have a clean menu bar once again.
All this really shouldn't be necessary and I definitively won't be paying for any Parallels products in the future unless they at least add the option to use monochromatic menu bar icons.
Click to expand...