I'd put them at both ends of the all the colored tracks as you never know when you may want to use them as temporary storage when you are breaking up a large train to sort or making up a long train to depart. I've learned the hard way to never assume you will have an engine or tender with an automatic coupler at that end of a stretch of track - you may very well find yourself in the middle of a consist. Now if you are never going to be running that long of a train then disregard.
-Greg