The default folder icons in OS X Yosemite and El Capitan, as in macOS Sierra, are a blinding shade of blue. You can easily replace them with darker versions of the originals by downloading this set of replacement icons and using the free LiteIcon utility to install them. Instructions are provided in the ZIP archive, including instructions for temporarily (and I think safely) temporarily disabling a new level of system protection in El Capitan and Sierra in order to install the icons.
The ZIP archive includes copies of all the default folder icons in recent versions of OS X and macOS, modified in Graphic Converter 9 to reduce saturation to -70. I have also provided darker versions of the Dropbox and Dropbox Public folder icons, and a folder-style replacement for the default Home folder icon.
Here is a preview image:
Technical note: If you want to add further similar icons, here is
the low-tech method I used when creating these. Use Find Any File to locate each
original icon; then Cmd-I to Get Info on the icon; click the small image at the
top of the Get Info screen, then Cmd-C to copy it to the clipboard. Then switch
to Graphic Converter 9, press Cmd-J to create a new image from the clipboard;
then use Picture->Brightness/Contrast/Hue to change the saturation to -70; then
Save in ICNS format to a convenient folder. Drag the saved icon into the
appropriate box in LiteIcon. Repeat until all the boxes in LiteIcon are filled.
Click Apply Changes in LiteIcon.
A real graphic designer would have done something a lot more careful and time-consuming to get the icons exactly right, and I hope that a real designer will do exactly that. But these are good enough for now.
Edward Mendelson (edward [dot] mendelson [at] columbia [dot] edu)