How to adjust the color of the folders in Finder in Yosemite?
Panic (panic.com) had a utility called CandyBar that let you change most of the default icons on MacOS.

Actually this was taken over by the Icon Factory (iconfactory.com)

It may still work but look at their site first. They admit it won't change dock idons, so don't even try that. If it does work on Yosemite you can change the default icons for a variety of things. Meaning if you change the default icon any newly created icons inherit the custom appearance.

Barring that you can copy an icon file that you like:

--get info on the folder
--click on the icon
--copy (CMD-C)

Then repeat that, replacing step three with "Paste (CMD-V)" on any icon you want to change. Only problem here is that it only changes one folder icon. So you gotta do that for every folder icon you don't like. On the up side it is fully supported by Apple and works a treat.