Removing coloured icons in Finder

If you use CandyBar, you can change the icons for system folders. Selecting a monochrome set of icons will make all the finder icons monochrome. Note that you cannot change the icons for applications from the App store without breaking their code signature, and thus their ability to update.

You can find the sidebar icons and most of the other icons to which you refer in the folder /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources

If one doesn't mind mucking about in system folders (something one should do with a great deal of caution), one might duplicate some of the Sidebar* icons and use them to replace the corresponding icons without the word Sidebar before them. I wouldn't try this without a bootable backup in place, however, in case I messed something up badly.