Support .clr files
Importing the standard Apple `.clr` palettes worked one time for me. It was the first thing I tried. After that it didn't work anymore, weather in the document section nor in the global section. Is there any way I can troubleshoot this or give you additional information?

Running OS X 10.10.5 (14F27) and Sketch  3.3.3 (12081) here.

Importing a `.sketchpalette` works great by the way, also clearing palettes.
